Just learned this one, and am going to be waiting for a chance to use it:
É cor de burro quando foge.
“It’s the color of a burro on the run.” (Literally, “when it flees.”) In other words, a color that defies description. This reminds me of a friend who dyed her hair and called it “a color not normally found in nature.” She doesn’t dye her hair anymore, alas, so I can’t use this phrase on her.
I’ll keep looking.