This is how JA+ does it: -
The actual effect itself is done by the saber color. There's color1 and color2 which are console commands (in reality they are cvars). So you could rotate through them with: -
/color1 9
/color1 10
etc.
Color1 refers to your first saber and if you have dual color2 does the second. For you actually change the color itself you can then use cp_sbRGB1 and cp_sbRGB2 for the actual color number (like you'd get from photoshop) or: -
sabercolor 1 20 30 40
where the first number is the saber you are coloring, the second is the amount of red, ranged from 0 to 255, and the next two are green and blue (at least, I hope its in that order!). Note, this won't do black itself, which is achieved by color1 11 & color2 11 - being a literal color. That means you cannot have a black saber with an effect.