After some research…
Steven would have either a turquoise aura because of his compassionate nature (I mean, he gives the player dozens of items throughout all the games he’s in) or a royal blue because of his chivalrous attitude.
Riley would have an Indigo Aura because it relates to the ideals of a third eye and Riley is able to sense things like auras
Cynthia would have a violet aura because of her sensitivity and wisdom she gifts to Steven and Riley but they often ignore.