Idea: use N noise fields and set the color based on which one is highest. N can be any number but in this demo I picked 4.

The sliders let you adjust bias B and multiplier M. Higher values make that color show up more often. You can also adjust the noise frequency F. Higher values make the features smaller.

I was expecting B and M to behave differently but they seemed to have pretty similar effects. The combination can be interesting though. Try setting M to be very low and B to be high.