The weird rainbow effects on Sonic's waterfalls are NOT due to the properties of CRT, but a result of the Megadrive's awful composite encoder. Connect the screen through a RGB cable cable, or composite through a 32X, and the resulting image is much cleaner.

Someone created a whole subreddit a few days ago to analyze the effect and post comparison pictures: https://www.reddit.com/r/Fuckingwaterfall/

In that Imgur link I gave, a comment by “Illithidbane” linked to this YouTube video, which is all about the waterfall and RGB vs. composite: <https://www.youtube.com/watch?v=x0weL5XDpPs>