"EEEEEk danger warning Will Robinson " You probably can find a paint for the top of a softboard but I think you need to consider a couple of things.
Some paints if they get through the outside material of the board will dissolve the foam big time.
Since the top of the board is designed to bend and have textured surface for your feet to grip a deck painting has two disadvantages , wherever you paint will be smooth and therefore slippery and whenever you put your foot on the painted area the softtop flexes ad will quickly crack off the paint!
Hopefully you will learn quickly and advance to a hard board soon
A product that is often used is the POSCA pens and then overcoat with a non dissolving type clear spray. Will it work for a foamy ? I don't know

I've taken up troll hunting just for fun, instead of a rifle I'll just use a pun! 冲浪爷爷