1 Dwalin: I thunk 'tis gettin' lighter. We moost be approochin' thu ind o' thu toonel. {translation: I think it's getting lighter. We must be approaching the end of the tunnel.}
2 Draak: Yes. Light mess with Draak heat sight.
3 Mordekai: "Heat sight"? Wait a minute. You have infravision? You can see in the dark? Why didn't you tell us?!
4 Draak: Not same as see in dark. Bare rock not show heat...
4 Mordekai: Don't give us your primitive pseudoscience excuses!

Again 1st Edition Advanced Dungeons & Dragons comes to the rescue. The 3rd Edition rules have some newfangled thing called darkvision, which apparently allows those who possess it to see in pitch blackness exactly as they would be able to if there was light. Yeah, yeah, I know it's fantasy, but that's just lazy. We had a perfectly good thing called infravision that worked with heat signatures, and was cool because it was different and based on real-world physics and geeky.

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2017-08-23 Rerun commentary: The new 5th Edition rules change darkvision to only allow vision in the dark equivalent to seeing with dim illumination, and only in shades of grey - no colour. This is a kind of halfway-house approach between the previously cool and the utterly silly. At least it's not as crazy as being able to see in full colour in the dark.

And yeah, it could just be magical and the way it happens to work, but it doesn't need to be magical.