I want to tint piping gel blue to use as a waterfall decoration. Do I tint it the same way as buttercream? Can I spread it on top of a buttercream cake? Does the buttercream cake have to be white or blue for the piping gel to look like blue water? Thanks!

Just put some blue color gel in there and stir. If it is on white buttercrea/fondant it will stay that blue. If you don't mix it well, it will have streaks that are darker which looks cool for the water effect.

I've done it on plain white BC and I think it turned out well. I like Loucinda's idea of keeping streaks of color in, I'll have to try that next time!

I also pull up some of the white bc from underneath for whitecaps.