I have tried latex paint on other things in the pond. Didn't hold up very
well under water. I've heard of others sprinkling dry sand or small pebbles
onto the foam while it's still sticky.
I used Great Stuff when I built my waterfall and just left it exposed. It
looked pretty bad for a short while, but sunlight quickly darkened it and
algae and moss have completly covered it over, can't see the foam at all
now!

Paul