Hi,
this is what I would do if you want to make just 1 paint spot: you could mask a cicrle on a layer, animate its scale from 0 to 100, add the turbulent distortion effect (so that it doesnt grow to even) animate the evolution, lower the size of the effect. Then add roughen edges effect, (maybe twice, 1st one larger than the 2nd). And there you go.
(instead of the turbulent distortion effect you can also animate the mask for more control)
Greetings
NoiZ