Nice explanation Dan. Really, if he wants other colours he should be fine.
Basically, the variables can either go up to one or to 255, depending on which you want.
If you're using one:
The first number is how much red is in the colour. This must be below one, and if there is no red then you want to put 0.
The second is how much green is in the colour. Same rules apply.
Third is blue. Rules still apply.
The forth is how transparent/opaque the colour is, 0 being invisible (or close to it) and 1 being fully opaque.
If you're using 255, it's pretty much the same as if you're using one, but instead of ranging between 0 and 1, you're ranging between 0 and 255.