I don't know a lot about the plot of this book as it has been quite a while since I last read it. But I do now that it was set in the space age, where everybody could travel between planets. It also had something to do with searching an mysterious, indescribable color (the 8th?).
There were aliens that were not able to perceive color who maintained the spaceships. A characteristic I can quite clearly remember is that these aliens (called lhari/laari?) would often get burned when they picked up a glowing tool, as the couldn't see it glowing.
At some point the protagonist undergoes a surgery to look like these aliens to do something they wouldn't allow humans to do. Maybe travel somewhere or spy on them?
I read it maybe 4 years ago, but it was an old book I picked up at a garage sale. I cannot name a specific time when it was written, but my best guess would be 1970-1980.
If I happen to remember more things, I will make sure to edit this question.