Adjusting colors besides RGB with Curves?

If you are not completely soured on using Curves (because the Info Panel will serve as a guide) and the color you want to replace has a scale of values rather than an overall flat tone, you may consider doing the work in Lab Color. There is a sound reason for it.

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 also introduce unwanted color variation within the selected area, as indicated by the arrow above. Lab Color mode on the other hand, avoids this problem 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.

First, create a precise color swatch.

1. A precise replacement 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 above (sans the gray line 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.    

Next, 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 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 Pigment/Ink % to flip the reference gray scales. 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.

If you have a color wheel you can generally see what primary and secondary colors make up other colors and that helps when adjusting colors with curves.

I like to use a hue/saturation adjustment layer, with that tool you can change colors without using a mask and target specific hues. Furthermore once you have selected your target range ie yellows, cyans, greens etc you can refine down precisely which colors will change. You do that with the slider at the bottom of the graphic below

You wrote you don't want to just use Curves. I assume you do not want to use Curves without the precise guidance of the Info Panel.

If the color is of a singe value and can be Selected using Color Range or one of the the Selection Tools, its components can be identified in the Info Panel. Then, for example using RGB Mode, and choosing Curves, that selection can be changed to a color with a different RGB composition. Choose a replacement color and jot a note of its RGB values.

Click on the selected area in the image with Color Sampler tool. Its separate channel values will appear in the Info panel. Choose Curves, hold down the Cmd+Shift keys and click the Color Sampler marker in the image. Its current value will appear on the curve of each color channel. Look at the Info panel. Use the keyboard North or South arrow to move the current value of each channel, as seen in the Info Panel to its new value, slowly changing in one increment steps.

southwestform  wrote Using Curves to drag up or down to remove Red/Cyan, Green/Magenta, Blue/yellow is pretty clear, though what do you do if you have a color that needs to be removed/manipulated that is not included in one of the previously listed colors? Ex: brown, orange, gold, pink, etc. Thanks in advance.

You may want to take a look at Camera Raw filter. It has a very nice option in HSL, where you can set your cursor above the color you want to subdue, and then just drag or use the scroll wheel to lower its saturation. You don't really have to know what color it is.

Those are all just generic names for mixed and muted colors. You need to consider what primary colors they have been derived from. Gold is a dark yellow, brown even darker with a slightly higher red content. And so on..

Curves is a pretty drastic tool for subtle color corrections, and not easy to target precisely. It can be done, but you need a steady and careful hand and a good eye. You may want to look into Selective Color or Hue/Saturation. And of course there's Color Balance.