I've never heard of even black material getting too hot on a swimstep. Seems like it's always wet so never gets hot.
Someone on this site just complained about sun burns on a Seadek swimstep recently. Something about water bottles acting like a magnifying glass?
Also, the peeling back up action your describing isn't really a function of the Seadek, but the PSA or adhesive used to stick it on the boat. As long as your putting it on fiberglass, I don't think it'll be a problem.