Alternate costume

In the SSB series, each character can change his/her outfit's color to advoid confusion if he/she is facing the same character they're using.

SSBM

The player can chose to control Popo or Nana and have the other Ice Climber follow you.

One of Mario's costumes makes him have Wario's (original) outfit.

One of Mewtwo's costumes makes him look light green. In the 2nd generation of Pokemon games and onward, this was Mewtwo's "shiny" color.

One of Peach's costumes makes her look like Daisy.

All of Kirby's and Yoshi's costumes are shown in their video games.

SSBB

Squirtle is the first character to have a confirmed Alternate Costume (his shell becomes lightblue).

One of Mario's costumes will be Fire Mario.

Link's black costume has been altered from a black tunic into Dark Link.

One of Meta Knight's costumes will be Dark Meta Knight.