When you make radio waves, you're charging already existing electrons in the air to make them wiggle. As I understand it, photons are emitted by electrons when those electrons wiggle in a very specific way, so when you make light you have the added step of inducing photon emission in the electron. So like instead of wiggling the electron the right way you have to get the electron to release a photon that's wiggling the right way.

This is what diodes do. For a while it was really tough to manufacture diodes that produced blue light. I assume this has something to do with accuracy of manufacturing processes.

Now we have smart LED light bulbs and you basically can set them to produce arbitrary colors of visible light lol. So I guess we're there or close enough to there already.