Do it with the GIMP → Overview of Features → RGB and HSV modes → What are RGB and HSV?

2.3.2. What are RGB and HSV?

RGB

RGB stands for Red-Green-Blue.
They are additive light components from which one can form every visible shade.

HSV

H = Hue : A unit that specifies the "colour" of the colour
S = Saturation : Specifies how much black, white or gray is mixed with the colour.
V = Value : Corresponds to Brightness.

Written by Shlomi Fish