If they're painted, (you can scrape off the color) I wouldn't trust them, if they're molded with the colorant in the plastic melt, they should be fine in saltwater (or fresh). Most of the plastics that you have in your tank (powerheads, locline, etc.) have colorant added to them either in the molding process or as a masterbatch from the plastic resin supplier. None of these things leach anything into the water column. Once thermoplastic is set, it is inert and does not change under normal conditions, that's why so many people scream about plastics not being biodegradeable. If you drop a plastic widget in the ocean today, it will still be there 100 years from now, looking just like the same widget.