I think most fan art that adds a suit to the original five becomes yellow whether the occupant is Hikari (post #219660), Maya (post #298935), or Mayumi (this pic). Actually I think this girl is psuedo-cannon in that she was a pilot in the "Rasing Shinji" game.
Yellow would be fine. The real problem is that this drawing is traced on official art (eg http://xoomer.alice.it/evaitalia/dvd4.jpg - does anybody have it in higher quality?). I can't believe I'm the first to notice!
They state it's a new character for the second Rebuild of Evangelion movie: In the super-deformed Evangelion game, the player collects prizes by passing levels. The final prize received from the last level is a character picture of a brunette girl with red-framed glasses named Mari. The description simply reads "Mysterious girl."