Evaluating Colors
The human eye perceives color in terms of three characteristics of hue, saturation, and brightness model called HSB. The color monitor uses a different color model called RGB because images are displayed using varying amounts of red, green, and blue light. When evaluating images, you can think in terms of these two models one to evaluate colors and the other to look for color casts.

To evaluate colors in an image, think of them in terms of hue, saturation, and brightness because these three aspects can be adjusted.

Hue
Hue is unique, it is the actual color, as measured by its wavelength, while the other two characteristics saturation and lightness modify the hue in some way. The hue can be red, orange, yellow, green, blue, purple, or any intermediate color between those pairs.
Saturation
Saturation, sometimes called chroma, is the strength or purity of the color. If you adjust saturation through its entire range, colors go from rich and vibrant to dark gray.

Lightness
Lightness, also called luminance or brightness, is the relative lightness or darkness of the color. Brightness is reduced by adding black to the color mix and increased by adding white. If you adjust brightness through its entire range, colors go from white to black. This is the only one of the three color attributes that gray scale images have.

As you decrease saturation (top), colors become muddier and finally gray.

As you decrease lightness (bottom), colors become darker and eventually black.

Color cast
A color cast is usually caused when one or more of the three color components (red, green, and blue) are too high or low over the entire image. This can be caused by not setting white balance correctly, by photographing a scene illuminated by more than one type of light source, or even a subject picking up reflections from a colored surface.

Color casts are very noticeable when shooting during sunrises and sunsets but there we usually like the effects. It's easiest to identify a color cast by looking at areas that should be neutral white or gray. If these areas have any colors mixed in, the image has a color cast that you should remove.

A neutral color has equal amounts of red, green, and blue and appears as a shade of gray.

Pure white areas should have R, G, and B settings of 255. Pure black areas should have R, G, and B settings of 0. Regardless of which neutral tone you are examining, if one or more of the RGB values is higher or lower than the others, these tones won't be neutral and will have a color cast.