Just be sure to avoid colors getting muddy when mixing a lot by using "replace color" (to control saturation or whatever) on the ones you want to pop a little, or by mixing in some more saturated red or whatever with the 50% or 25% brush opacity maneuver. Also, after applying a stroke you can fade it and control the opacity using cmd+shft+f (useful if you wanted slightly more red or less red in a color). You can also do stuff like paint bucket a color over another color and use fade to mix them. Or you can fade after using replace color to lessen the effect.