You cant change it for just one character. Vegeta, Goku, Gohan, Trunks and Piccolo use the same aura texture (white) You can change that one, but it would change it for every one else above.
But heres how you do it since LionHeart didnt help at all.
The turbo aura is in the models folder, Aura.mdl. If you have Half Life Model Viewer you can open the model that way and export the texture that way, edit it however you wan to, import the texture and save it.