You could create a few different color schemes, and then use a slider to allow them to select between Look 1, Look 2, Look 3, for example.
Depending on how complicated your template is, you could use multiple instances of the Fill effect, make duplicate versions of a few pieces, of the whole comp, or use Master Properties. Whatever.
Create a slider somewhere, send it to the EGP, and limit it to the number of color options you want. (1-3), for example.
On the opacity for Look 1 (or layers, effects, etc., depending on how you built this thing), you'd add this expression:
var slider = pickwhip to your slider;
if (slider == 1) 100 else 0
Change the 1 to a 2 for the next look, and so on.
It should be noted that this will only detect whole numbers, so if the slider is set to 1.3, for example, it won't detect it. There are some additional functions you can add to the expression if you need it to be more "aware", but with only a few options, this should work fine.