Let's just say we're talking about one mad genius here, and not a group, and the destruction of the earth is his brainchild. Makes his beret easier.
He'd need a space suit. An actual space ship like a shuttle would not afford maximum viewing angle, nor that oh-so-necessary "road feel" that motorcyclists love.
His beret would be worn, I should think, on the outside of his helmet.. how else could he be recognized as the artistic genius he is.. without it, he'd get written off as some ordinary maniac mastermind.
Therefore his beret would have to be large enough to fit atop a space helmet without looking ludicrous. It could be attached with any old glue.
Colors? I think a bright red, stupid ultimately french-looking beret would be optimal. The more tasteless the better.