is there a colour jitter brush in fire alpaca or something similar?
Hello! you can actually make your own color jitter brush in Firealpaca.
I don't know exactly what you would use it for (maybe just put that in your ask next time, it's easier for repliers usually) but at least 2 types of custom bitmap brush have this: the bitmap and scatter rbsushes that you can create yourself.
You can find the colour jitter at the bottom of the brush edit window:
Experiment with this color hitter option (and possibly hue jitter as well) to find the brush look you want.
Sorry for the late reply nd hope this helps!