At this point, we’re still four years out from Kara Zor-El’s debut (in Action Comics #252), and three years away from the “Super-Girl” Jimmy Olsen creates with a magic totem in what would seem to have been a dry run for the character (in Superman #123). Kara’s outfit featured a blue skirt while her precursor had a red one. (In Action Comics #156, temporary Superwoman Lois Lane had it both ways; red on the cover, blue inside the book).
Actually Kara had it both ways, too. In her first appearance, a preview at the end of the Superman story in Action #251, she had on the red skirt:
And although it was blue in her full-fledged debut in the next issue, they often goofed for the next year or two, as in this cover from Action #262:
Personally I thought the red skirt was better. And I'm sure if anybody wrote to Mort Weisinger about this "boo-boo", his response was probably that like all women, Supergirl had more than one outfit.
Update: Another example of the red skirt: