Q: Say I have someone wearing a single coloured top, in this case black and I wanted to change it to another colour something brighter like blue or white. How would I go about this?

A: Changing the color of an object in Photoshop is relatively easy if the original color and the color you're changing to are both about the same lightness. The farther you have to go in changing the lightness, the more difficult it becomes.

One of the most challenging retouches I've faced along these lines was to take a photograph of a model wearing a white tank-top for a swimwear catalog and make her tank-top black. Doing this, and making the result look natural and unretouched, took many hours.

Here's one way to make a light object darker:

Create a new layer. Fill it with white. Set its Mode to Multiply. This will make the layer seem to vanish.

Now, switch to the layer underneath. Examine each channel in turn, until you find the channel which offers the greatest level of detail in the object (in this case, the eye) you wish to darken. Make a selection around the object, and copy it.

Switch to the while layer. Go into each channel in turn and paste that object into the channel. This will have the effect of darkening the object (because the layer is set to Multiply) while preserving fine detail. (Note: If your image is CMYK, not RGB, you may not want to paste into the black channel, depending on the quantity of black generation you want.)

Now switch back to all channels. The object won't be the right color, but it will be darker and will still preserve detail. If necessary, adjust the layer's Opacity if the object is too dark.

The color of the object can be changed by any of the normal techniques:

Hue/Saturation, painting or filling in Color mode, etc.

You can put the Paint Bucket (or any painting tool, including the Paintbrush) into "Color" mode. That will change the color without changing the shadows or detail.

Hope that helps.
--Tacit

Monitor Calibration

After many years with Photoshop and other design software, I still find it really challenging and difficult to scrutinize the various aspects of color profiles. Maybe many people will give better idea about the correct method for dealing with a deceptive topic like color profiles. However, I will mention some facts that I learnt during my work:

The first and basic fact that stands behind all implications of profiles is the following:

Waves of Light

RGB (Red Green Blue) are the waves of light (I even do not prefer to call them colors, as the case with printed inks). They are of additive nature, i.e. if the values of RGB are (0,0,0)-RGB then it is Black or more precisely a BLACKOUT.

Practically (0,0,0)-RGB means that no light is coming at all or the bulb is OFF. On the contrary (255,255,255)-RGB or more precisely (100%,100%,100%)-RGB will yield WHITE light, given the medium is originally black like a dark room or even a COMPUTER'S MONITOR.

Printable Colors

On the contrary, CMY (Cyan Magenta Yellow) are the inks which reflect light, so they are the printable colors. CMY are of subtractive nature, i.e. if the values of CMY are (0,0,0)-CMY then actually they represent no color at all. If no ink was printed on a white paper, then the area is white. On the contrary (100%,100%,100%)-CMY will produce Black (in theory). RGB is the complement or mirror system of CMY. For example, in theory (20%,30%,40%)-RGB (or about (51,77,103)-RGB on 256 scale) should be printed, in theory as (80%,70%,60%)-CMY.

Grays & Blacks

However, for the reason of impurities that can be found in those inks, (100%,100%,100%)-CMY may only produce muddy black, or more visibly a very deep brown. So, Black ink is printed separately to compensate for printing three inks to get a single faux Black. The idea of using the Black, while not being a basic color, has led to the ideas of using further spot colors (see below) to compensate for faint tints in printing.

This leads us to a fact that the only color which has equal values in both RGB and CMY systems is the (50%,50%,50%)-RGB (i.e. (127,127,127)-RGB) and (50%,50%,50%)-CMY which is the Gray color. But we have to understand that the first Gray is a LIGHT that may illuminate darkness, and the second gray is an ink that can reflect light.

Adobe Gamma

This is why we calibrate the screen using a Adobe Gamma. Adobe Gamma will help us realize a suitable balance between the gray we see like a light and the gray we see printed; thereby, define a point of reference for what we call it Gray in RGB system. This point of reference will, theoretically, lie in the middle of RGB scale. However, practically IT WON'T. So, the Gray for a certain monitor is (127,129,128)-RGB and (128,127,130)-RGB for another monitor.

The shifting percentage is the calibration done by the color profile. So, we will be in need of a printer calibration (profile) that will respond to the RGB reference point. That profile will interpret RGB values into printable CMY values and should be able to decide when to use the Black ink (in simple printers), and/or other spots like Orange, Green, Light Cyan, Light Magenta, (in more complicated printers), such that it prints the widest possible spectrum.

For the reason of the nature of light, RGB (lights) will NEVER be the same colors printed on a printer, however, they will closely resemble them, in certain lighting conditions. Other reasons why printed colors cannot be the same as the monitor colors could be the paper's native color, ink purity, paper's roughness, press blanket pressure, etc.

According to the above, I advise the following:

1- MONITOR CALIBRATION:

Calibrate your monitor using Adobe Gamma and pay a lot of attention to see which gray is the best neutral gray. If you don't know how calibrate your monitor, hire someone to do it for you! It is a crucial step.

2- WHICH PROFILE?

Some people will advise you to use so and so. The golden rule DON'T trust anyone nor trust YOUR instinct, nor trust ready-made profiles (that were done in laboratory ideal conditions)! Trust only real experiments!

Remember that profiles are nothing but calibration tables. So, if you refer to the scanner's manual and you were advised to use a certain profile, DO NOT quit trials with other existing profiles you have. Take some time with your scanner and printer and make various experiments.

First try NO PROFILES. Then try with the profiles included with the scanner and the printer. Then try some other well-known profiles. Eventually you will reach at satisfactory results.

Remember: what you see on monitor (soft proof) is always brighter than hard proof. If you suspect scanner's profile that ships with the scanner, just use AUTO COLOR feature in Photoshop, then do a visual comparison with the scanned image. Use other tools (as needed).

3- OFFSET PRINTING:

Having calibrated your monitor using Adobe Gamma, you have to stick to working in CMYK space. Photoshop's native CMYK will always make you sure that the color values input by hand will remain to the end this way! So DO NOT USE ANY PROFILES for offset printing. Discard all profiles, and correct colors manually before you ship your final publication to separation shop.

However, you will need to decide the Black generation process. Black generation is crucial for good offset printing. UCR will be used to compensate the gray shades composed of equal amounts of CMY with a black tint only. Whereas, GCR does wider substitution for different amounts of CMY.

Some paper types will highly absorb ink and make the PostScript dot more spreading, and therefore; we should decrease the ink, hence, increase the Dot Gain. (Refer to Photoshop online manual for more information). If you want to output CMYK image to a printer, then it is better you choose Printer's Color Management. But when the matter comes to output separation films you actually need NO PROFILE tagging if you worked in CMYK from the beginning to the end.

4- CMYKOG and CMYKLcLm:

CMYKOG (Cyan-Magenta-Yellow-blacK-Orange-Green) and CMYKLcLm (Cyan-Magenta-Yellow-blacK-Light cyan-Light magenta) are two spaces of hexachromatic nature (i.e. six colors process). Both spaces are used on the Large-Format printer.

CMYKOG (erratically called this way though sequence of lithographic plates puts Black last) is used in offset printing and pigment-based inks, while the CMYKLcLm is used with dye-based inks on Large-Format Printers. Apparently, color profiles are indispensable to coordinate the roles of all the six inks.

The idea behind the six colors is that additional colors are used to output faint tints. Just imagine a green of (6-0-6-0)-CMYK to be offset printed. It will look like a faint thing! What if we used a 20% tint of a premixed (spot) green (30,0,30,0)-CMYK, that would blend seamlessly with the image and the screens output in the lithographic films was 20% density? Of course this would be delightful, and there will be a chance to simulate more of the RGB space. Details of faint tints substitution are much like having a Transfer Function (like that used in Photoshop) that will tell the printer about the generation of the dot currently in print.

In case of color separation, lithographic films will be generated according to the real pixel values assigned by the designer (in Photoshop's CMYK space) OR the SHIFTED pixel values generated by Color Profile. Most of us will NOT be 100% sure on which image setter the artwork will be separated, so there should be some machine-independent procedure for the color profiles. Native CMYK space of Photoshop is always excellent, if and only if we determine how much the dot gain in the conditions of printing press.

FINAL WORD:

The designer is completely responsible for any negative changes that occur because of downsampling from RGB to CMYK. Before separation, the designer should have the client's cursive approval on the final hard copy of the design.

Mohamed Al-Dabbagh
Senior Graphic Designer

Correction:

In reference to #4 above, here's what Scott Donkin, Colour & IT Systems Specialist for On Demand Solutions in Chatswood, NSW Australia says:

This is not correct.  CMYKcm is not a hexachrome process whatsoever.  The function of light cyan and light magenta are purely there to offer smoother transition from light to dark densities of ink as laid down by the printer.  The additional light inks do not offer a larger gamut.  In fact, a CMYKcm printer is not a “six colour printer” – it’s a CMYK four colour (yeah we spell funny on this side of the planet ;)  ) printer with a couple of light inks.

Scott