Generally, as a rule, I always hyphenate words to make them into a single adjective, so I've been putting "blood-red", "forest-green", "royal-blue" and the like, but the moment I typed "royal-blue", my instincts kicked in.
It didn't look right to me. Perhaps it's because this is an official colour or the fact that the word "royal" is unrelated to blue, but I'm not sure.
What do you guys think?
P.S. I'm using British English, if that helps.