H: Hue: This determines the color. It's a from 0 to 36, in degrees.
S: Saturation: This is the depth of the color in Percent, with 0 being plain grey, and 100 being the deepest color. This is what the slider I have highlighted in red is.
B: Brightness: How bright the color is, with 0 being black and 100 being white.
R: Red: Amount of Red, from 0 to 255
G: Green: Amount of Green, from 0 to 255
B: Blue: Amount of Blue, from 0 to 255
The key to getting good earthtones, especially browns is to desaturate (def: lower the saturation of) colors. To get to brown colors, which can be hard to notice on the color picker, you go half way between Red and Yellow on the left, and desaturate the color to somewhere between 25% and 75% depending on how desaturated you'll get varying degrees of browns. You can also change the hue by a few degrees, and adjust the brightness to fit the map.