Check out Plasti Dip. It's available in a few different colors as well as a spray can (nice for use with stencils).
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I did some on-line searching and I wonder if what you are thinking of is Trident's U-Mark-It paints. They are available in a number of colors, even glow-in-the-dark, on Trident's website.They used to make a rubbery paint called "U-Name-It" that was great on wetsuits. I don't think it's still available (if it is please get me a bottle of white and a bottle of blue).