Some closed-cell foams that cannot be colored with traditional dyes can be re-colored with permanent marking pens. But it's a pain.

Most such marking pens consist of a tubular plastic barrel filled with a fibrous wick, that has been soaked in a colored, solvent-like liquid.

If you could buy bottles of those solvent or alcohol-like colors, it would (in many cases) be a lot more convenient to dip an entire foam chunk into a bottle of the same liquid. Those colors must be available. Marking pen manufacturers use them. What are they? Where can I get them?