How to change saturation on a curve in RGB mode, in Photoshop?

Question

In Lab mode it's easy to change saturation independently of luminosity, by using curves on the a and b channels, but how can you do it in RGB mode? In RGB mode it's still easy to change luminosity independently of saturation by chosing luminosity layer blend mode. Photoshop also has a saturation layer blend mode, however, how do you get a saturation map to blend with?

Of course you can use a duplicate layer that you do your adjustement on, and using saturation blend mode you'll only get the change in saturation, not in luminosity, but unlike Lab mode, you can't grade on a curve, so to speak. You can't put a steeper curve on the saturated colors while keeping more neutral colors closer together (opposite of vibrance, basically), or can you?

Is there a way to get a saturation map from a Lab copy of the document, and use that somehow in RGB mode?

Answer 1

What you describe, editing saturation curves in RGB mode, may not be possible. However, it is possible to enhance a few colors in an image with the curves tool.

The way I think of saturation, which may be incomplete or even just plain wrong, is decreased saturation is more gray, while increased saturation is less gray.

In RGB, the gray tones have an even amount of each of red, green, and blue. This is often used to "white balance" an image. Several points are selected that are intended to be neutral. Curves are adjusted to make the RGB values approximately equal. Do this through out the image at various intensities to remove color shifts.

If, instead of neutralizing colors, you want to emphasize colors, you can select areas of an image you want to enhance. Then adjusting the curves so that the RGB values are farther from neutral. It helps to know the RGB values of some of your favorite colors, as well as the inverse of RGB: Red-Cyan, Green-Magenta, Blue-Yellow.

For example, to emphasize "blue" tones in part of an image, select a few points. Then for those points, reduce red a bit, maybe leave green alone, and boost blue. Since you likely don't want to give the entire image a cyan-blue cast, make small, limited changes to the curves around the regions of the selected points.

Although enhancements to multiple colors can be done within the same set of curves, it's easier to work on each color in its own set of curves. When using curves this way, I usually use color blending modes to avoid letting luminosity get out of hand.