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One of the limitations of working in RGB is that alteration of color changes the tonal scale of the image, as well. In order to change the color while not changing the tonal scale and local contrast -- the texture that defines the dimension within the piece -- switching to Lab Color Mode is a good choice. If you are not familiar with Lab, be prepared: although the L (Lightness) channel Curve adjustments will be familiar to you, the a channel, which controls Magenta and Green and the b channel (which controls Yellow and Blue) are far different from RGB curves.
Here is a very brief introduction to Lab Color. It is taken from notes I prepared for my students and accompanied the lecture.
How to Obtain a Precise Color While Maintaining Object Modeling
Changing the color of an object when in RGB or CMYK mode affects each Channel’s tonal scale, as well. Unfortunately, a tonal shift may introduce unwanted color variation within the selected area, as indicated by the arrow above and a change in tonal modeling. Lab Color mode on the other hand, avoids these problems because it stores tonal modeling of an image in one Channel (L) and its colors (and degree of saturation) in two other Channels (a and b).
Each Channel’s corrective work is done in Curves and assumes that you are working with an image that has already achieved gray balance.
The L (lightness) channel stores only the tonal modeling and is presented as a somewhat lighter black & white version of the image.
The a channel stores Magenta & Green color information.
The b channel stores the Yellow & Blue color information.
An object’s color in these channels places gray (no saturation) at zero value and its degree of color saturation by its distance from zero. Warm colors (magenta & yellow) are given positive numbers and cool colors (green & blue) are given negative numbers,
Middle intensity gray may have the designation: L(50), a(0), b(0)
Deep purple (a mix of magenta and blue) may have the designation: L(26), a(25), b(-22)
L(26): L(26) is dark gray. Black would be (0) and white (100).
a(25): A magenta value. The plus number extreme is (127)
b(-22): A blue value. The minus number extreme is (-128)
How to obtain a precise color and maintain object modeling using Lab Color mode:
1. A precise color is often specified in terms of its PMS number. To place that PMS color in the Foreground Color box of the Tools panel, click on the Foreground Color and in the resulting Color Picker, choose Color Libraries > Pantone Solid Coated.
2. Rapidly type the Pantone number even though there is no field to enter it. It will display the color. Note also that its Lab values will be displayed. Make a note of them. Click on OK and the color will appear in the Foreground Color box.
3. Create a small file similar to the one shown below (sans the gray surround). When creating this new file, set the Background Contents field to Transparent so that the hole in the center of the swatch is empty, not white.
Change the Object’s Color to Match the Swatch
4. Open the file containing an object in which the color is to be changed to match the swatch; then change its Mode to Lab Color: Image > Mode > Lab Color
5. Drag the Color Sample file over the object. It will become Layer 1.
6. Change the Info Panel to read Lab values in the Info Panel by clicking on its eyedropper and choosing Lab Color.
7. Choose the Color Sampler tool and click inside the Sample where the current color shows through the hole. Current Lab values will appear in the Info Panel.
8. Turn the Eye off on Layer 1. Move to the Bottom Layer in the Layers panel and create a Selection of the area to be changed. Then return to the Color Sampler tool.
9. Choose Image > Adjustments > Curves and then Cmd+click on the Color Sampler register mark. This will place a dot on the L curve. Its value will appear in the Info panel alongside the L value in Step 7. They are the same now but will record any change.
10. Use the keyboard arrows to move the dot vertically to match the L value that was recorded in Step 2 (your new color value). Use the Info panel to monitor the move.
11. The a and b channels are adjusted differently from the (Lightness) L channel. In Curves, switch from the Lightness channel to the a channel. Using the center point as the pivot, rotate the curve line to match the LAB value that was recorded in Step 2: counter clockwise to increase it, clockwise to decrease it. Repeat the process in the b channel. Use the Info panel to monitor the moves.
Note: When rotating the a and b curve lines:
the center point must not shift or gray balance will be affected.
the line must not bend or saturation relationships will be affected.
Also: To change a plus value to a minus value or a minus to a plus value, first transpose the curve line…
… then rotate the curve line to adjust the value.
12. To confirm color match, turn on the eye adjacent to Layer 1 in the Layers panel. After confirming the color match return to RGB or CMYK Mode.