The last time I tried that, I went with a pretty simple contrasting color for to the rest of the base decoration.
I decided not to overdo it and keep it a simple, single bright color because the way I made tentacles I figured less is more and a plain paintjob would complement the plain sculpt. Generally I agree with queen_annes_revenge, trying for a simple dark to like transition is not a bad idea.
Previously I got my pink tentacles by throwing black and red wash on a white base. It's messy, but has more texture:
This might be a valid approach depending on which colors you use and whether you're careful with the application of the wash (unlike me

).