Well, heat vision could simply be intense infrared rays. That is the portion of the e-m spectrum most closely associated with heat. IR is, of course, invisible to the human eye, but there are IR goggles.
It doesn't have to be IR. How hot something is is simply a measure of how fast the particles that make it up are moving. Anything that can get them moving faster will heat them up. Air friction, sound waves, intense light, physical contact with a hotter material... there are plenty of options. Specific materials respond to certain frequencies. Water molecules, for example, naturally move at a certain frequency. If you hit them with electromagnetic waves tuned to the same frequency, they start moving a lot faster. That's how microwave ovens work.
As for lasers, the reason you can see them in fog is that they're generally tuned to some visible portion of the spectrum. Move the frequency out of the visible range, and you probably won't see much, fog or no.
Of course, water does have a strong tendency to refract and scatter electromagnetic waves (which not only includes x-rays and other types of what we normally think of as radiation, but also visible light). That's why subs have to use sonar. A thick enough fog, and heat vision won't have much of a range. Even x-ray vision will probably be more limited than usual.
Assuming it is something like infrared radiation, it'll do to lead what it'll do to just about any other metal -- hit the surface and start heating it. Lead, as it happens, has a relatively low melting point (for metal, anyway). While it is impenitrable as far as x-ray vision is concerned, heat vision should have no trouble simply melting it away or burning a hole in it.
As for what x-ray vision is, I haven't heard a single good explanation. Someone did tell me, however, that the further you go along the spectrum, the more likely it is that a given type of radiation will pass right through an object. Visible light will bounce off most things, but x-rays will go through just about anything less dense than lead. Go even further down the spectrum, and you find nutrinos, which tend to go through entire planets without stopping. ("Nutrino traps" are designed to slow them down enough to study. Basically, they take extremely sensitive equipment and put it in a chamber with extremely thick concrete walls surrounded by thick lead walls surrounded by ... all of which is located underneath a mountain. Every once in a while, a single nutrino, passing by with tens of thousands of others, will have been slowed enough by all this that the sensor will be able to detect it.)
So, x-ray vision could basically work along those lines. He shifts his perception so that he's detecting a different portion of the spectrum. The further he goes, the further he'll see. This wavelength will pass through the first layer and bounce off the second. This one will go through the second but not the third. And so on.
He wouldn't even have to emit x-rays or whatever. He'd just have to be able to detect the background radiation that's already there, the same way we detect visible light without emitting anything.
I don't know how much that really works, but that was the theory I heard proposed. I think it was a physics major/comic book geek friend of mine, but I'm not certain.
That doesn't really explain why he can still see color. If he was looking at a different portion of the spectrum, he'd be leaving normal visible light behind. Red paint, blue paint... it'd all be more or less the same.
I don't know if there is an actual answer to it. While looking for explanations is fun, not everything Clark can do is physically possible (in our universe, anyway). That's part of the fun of imagination.
Paul
