Oooh! Okay, cheers for the help!Well, the color of the Waterfall sprite will change after you subject still water to the edges of a half block gemspark block. So if you want to make a purple waterfall, you need to use Amethyst Gemspark blocks to do that, which areade from Glass and Amethyst gems.
To change the color of the pool of water itself though, you will need to use a fountain, particularly the Corruption themed one.