I know you were kidding, but let me counter that. There were numerous times in the various comic series, movies and television shows (both live-action and animated) when Superman/Clark Kent reacted to the presence of green kryptonite (by collapsing or feeling ill) without actually knowing any was around, which definitely seems to work against that hypothesis.
Maybe the sound of kryptonite’s radioactive decay affects his super-hearing in some way. Maybe there’s something about the wavelength of its light that causes his powers to short-circuit.
I take your point, but there are holes in comic-book physics that you could drive a Batmobile through. 
No argument there. And if you don’t like the reasoning they give you, wait six months and they’ll retcon it…
So, color is reflected light. Changing color would be changing reflected light. He wouldn’t need camoflage or to mess with someone’s cornea, he could just affect all light in an area so it does not reflect and the area goes dark.
Or gather light to form beams of various functions. He could be a light bender from Avatar.
If he is crafty enough then he could change reflected light selectively and convince criminals of some pretty neat illusions: “Your hand is on fire” “No it’s not” “Sure it is, look and see” Criminal looks at hand and sees flames licking up and down between his fingers “Yahh… my hand is on fire!”
I must ask—are there any (canon) limits to the duration of effect of his powers? I mean, if he changes something’s color to red, is it red forever, red until he changes it back, red until he dies or is knocked unconscious, or does the new color just “wear off” after awhile?
Speaking of Kryptonite…you might have some luck taking a sample of one of the other, relatively harmless varieties of Kryptonite, some of which I’d at least hope are more common or less valulable than one of the more lethal varieties, tricking Superman (or another Kryptonian) into picking up or getting near the “harmless” substance, and then turning it Green…or Gold.
Heck, would it be possible to create new varieties of Kryptonite in this fashion, by changing a sample’s color to something not previously seen (like purple, or orange, or the colour out of space)? Or would that just make the universe implode…a little more. [sub]Again.[/sub]
Radio waves are just another frequency. And what else is radar than a form of seeing with radio ?
Of all the things that Color Kid could possibly do with his powers, I’d think that reducing retail sales of fish would be pretty low on the list.
He was, actually, very briefly, because he was a member of the Legion of Substitute Heroes (an unofficial adjunct to the Legion of Super-Heroes who were rejected for powers too iffy), along with Infectious Lass, who accidentally infected him with Grandin Gender Reversal Germs. (She was powerful, but had NO control at all.)
He was cute as a girl.
(And Bouncing Boy’s powers are odd, not useless - they, even if you think the bouncing itself useless, involved increased durability, and demonstrated immunity to electric shock - and Matter Eater Lad, who could eat and digest anything short of Inertron, regardless of hardness or size…if he were evil, he’d be one of the most dangerous people in the galaxy.)
In the spirit of the OP ( since it’s mine), how would he be dangerous?
By eating Superman ?
“Mmmm . . . Kryptonians are so nice and crunchy.”
Getting back to the OP, I can imagine all kinds of nasty and cool and nastycool things I could do if I had the ability to shift energy wavelengths up and down.
I could, uh, colourize old movies!
Say, he couldn’t chance the color of a star, if he got close enough to it, could he?
And how precise are his abilities? Could he change the color of scar tissue, or tattoos? Could he make tattoos appear on a living being?
As an assassin, he’d be top notch. He’s well trained as a fighter, and when he goes for the kill, no armour (at least none that anyone short of a Kryptonian or Daxamite could move in) could stop him. If he wanted to kill you, he could, quite literally, bite your head off. Hiding’s no good - If he wanted to get at you, he could simply chew through you your walls.
And less direct damage… He could ruin economies (as there are still worlds in the UP whose economies are based on physical goods), he could cause famines, if he had a little time - he appears to have a bottomless pit for a stomach, given some of the stuff he’s eaten. Destroy Winath’s weather control devices. (In the continuities where they exist.)
Only two things have ever stopped him that have been shown - inertron, which he can’t get through, and the Miracle Machine, which he was able to eat, but was driven insane by the energies it contained.