Methods Of Changing Color

More control can be gained over an image by decomposing an image first and then manipulating the color channels in either RGB, LAB, CMYK, YCrCb, and HSV mode. HSV and LAB are my favorite modes for editing colors. The G’MIC plugin for GIMP gives you the ability to edit colors in all of these color modes.  

Hsv(Hue Saturation Value)

Adding more contrast to the hue channel will move hues farther apart, decreasing contrast moves hues closer together. Increasing contrast in the saturation channel. channel actually doesn’t add more color to an image as you might think. More contrast will make the areas that are most saturated even more saturated, and the areas that are less saturated will become even less saturated. The value channel is a black and white copy of the image. You can only change the tonal values of the image by changing the value channel. This is also true when decomposing to YCrCb and Lab. There will be one channel preserving the tones of the image while the other two let you modify the colors.  

LAB(Lightness A-color B-Color) and YCrCb(Luminance Red & Blue Chrominances)

YCrCb and LAB split the color into two channels that when changed will have little or no affect on the tonal values of the picture. You can add contrast to one channel like the b color channel to make the yellows yellower and the blues bluer.  Changing brightness on a YCrCb chrominance channelS will have an affect on more hues and will be more of a drastic color change. It sounds complicated, but there really isn’t much thought process in any of it. You just experiment around with different sliders and see how they affect your image.  

Change individual colors

As Silvio mentioned in the comments Lightroom allows you to change the levels and saturation of individual colors. I use Raw Therapee to go through my raw pictures. It has a similar feature. I haven’t used it enough to be able to say too much about this feature, but it seems to be a powerful tool for changing the hue, saturation, or value of a specific color (red, yellow, green, cyan, magenta, and violet). In Raw Therapee this is done gradually, instead of using a threshold, keeping things looking natural when you edit color by color. G’MIC for GIMP has a similar HSV equalizer, but it  has a strict threshold instead of a gradual one so results look unnatural.      

Guidelines

Moderation

If all of the colors are overly saturated, then none of the colors will stand out. Changing contrast in the color modes talked about above can be used to selectively make part of the image stand out. Don’t overdue the color.

Create a mood

You don’t have to color balance an image to what is most correct. You should be changing the colors to replicate the mood you want to portray.

Warm vs cool colors

cool colors create a lonely cold mood while warm colors give an inviting feel to the scene. Contrasting subjects of a picture with warm vs cool colors can give an interesting affect.

golden hour

Often colors stand out the best at the time of day photographers like to refer to as the golden hour. See this article if your not familiar with the golden hour.

Change color locally

Using the techniques described above one can change the color on different layers, and then use layer masks to accent specific parts of a picture.  

Color Themes

There are different ways of creating a color theme. Some of these methods are complementary, analogous, triadic, tetradic. Here is a fun game about color themes.  

Further Reading:

• A website dedicated to the subject of color (Great site I am looking forward to reading through it)
• The G’MIC plugin
• Using Color in Photography
• Harvard Video about color spaces, how sensors detect color, calibrating devices, and other technical stuff.