The Colors of Space by Marion Zimmer Bradley

I never guessed that Bradley had done YA. I have to say that it isn't her strength, but it was all right. It came out in 1963 and it is a bit dated, but not badly enough to make it unreadable. Oddly, they keep talking about the aliens' "warp drive." And I know that was before original Trek launched. The plot is fairly standard, I'm trying to remember the title of one that is very similar - I'm thinking it is one of the less distinguished Heinlein juvies. Citizen of the Galaxy, maybe. Anyway, boy gets dumped into intergalactic conspiracy and saves the universe and gets the girl.

AND discovers that the evil aliens aren't as bad as all that, and is the agent of a new peace. I think Bradley was trying to dumb it down just a bit much. She has created an alien race so similar to humans that with a little plastic surgery our hero is able to pass as one of them. They can even share plasma - and they come from a different galaxy. And for a bit of a reach, the aliens are color-blind and don't know that their secret fuel is an unknown color that is not in the Roy G Biv visual spectrum. She tried so hard to describe this eighth color that I began to suspect that she had written the story on some kind of bet.

Obviously, this is not a Darkover book - but for all of her fans, she put in a Darkover allusion. The alien ship with our hero is forced to put in for emergency repairs, and one of the worlds almost within range is Cottman IV. That's it. They don't go there, but it is there, because as all true Darkover fans know, Cottman IV is the Terran name for Darkover.