The Spanish rubio (meaning “blonde,” as in the hair color) comes from the Latin rubeus, meaning “red”.
How did “red” come to mean “blonde”? In a world where everyone has very dark black hair… it’s easy to see how everyone could conflate blonde hair and red hair. The Romans didn’t know the Irish!
From the same Latin root, we get various English words including Ruby, the stone, and guess what color it is? We also get Rubric, which was originally religious directions that were written in… guess what color ink?