Encouragement, heck, it was Aeris' idea. And then once she convinced Tohru that Sephiroth's gloom and doom had to be influenced by the black clothing -- look how gloomy and doomy Cloud and Leon were, also wearing black clothing -- she had the World's Most Eager Helpful Puppy-Eyed Co-Conspirator on her hands, and then it was just a question of aiming her, so to speak. XD
I suspect Zack is going to talk them out of pastels for the moment -- "you've gotta work up to pastels, honestly; remember, he's been color-impaired all his life. So baby steps, a little at a time... how about starting with beige?"
"BEIGE?"
Sephmumble: "...beige isn't pastel."
Zack: "Baby steps, right? The training wheels version of color. And then maybe when he starts acquiring some rudimentary fashion sense he'll be eager to get rid of the beige too..."