Check this out: heat-sensitive glass tile. I may be the last person in the world to hear about it. Mom sent over a newsclipping and I investigated a bit.
The tile changes color according to temperature, so if it’s in your shower you’ll supposedly get a continuously changing rainbow while you scrub. The site’s kinda info-poor and they don’t really say how it works. I’d suspect some kind of heat-sensitive crystal, which also makes me wonder how long it’s going to last. (And what happens in 10 years when color-changing tiles become as dated as shag carpet? Can you turn it off and pretend it’s just plain tile?)
What they DO say, though, is that this stuff ain’t cheap. You can buy samples of the different tile styles, $33 apiece for a 4×4 tile (or nearly $300/SF). They also require you to agree to we-don’t-trust-you payment terms before they’ll mention the price.
I suppose if I ever get rich, I might give it a shot.