(+) Implement a color wheel/ gradient of sorts. Would allow a greater range for "tag" customization. Functionality of the 'E' key would remain the same, albeit the color lineup would be capable of being edited by the player.

That's close to downright impossible unless I create 16777216 separate images for each shape

Actually, possible not. There is util.DecalEx

if you make smaller sprays variation and ''straightline'' mod that would be very cool!

I know this thread's old but couldn't you just have any shape that's colored white and then have a function to color the shape before it's sprayed?