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What's the eggcrate for? I thought you were going bare bottom. If its for under the rock you could use StarBoard do a search for it you'll find it. You could even use acrylic. Or like everyone else said just buy black eggcrate. I'm sure the shop could get it or if not then fish factory always had it in stock
The fear is that white eggcrate leaches phosphates into the water. From what I've read on the internets, this is an issue with how styrene eggcrate is manufactured, so it shouldn't matter what color it is, just that it's styrene based. If it's made from acrylic, it should be fine, white, black, red, whatever.
That being said, if you really don't want to waste it, you might want to try Krylon Fusion spray paint. It's for plastics and isn't supposed to chip after it dries for a week. Or so the can I have says. lol. Not sure if that would truly seal in whatever might leach out though.