Generate random shootable spot on tube

Hi,

I'm trying to create a game in which you shoot a ball at a rotating tube. I want a piece on the tube to be white (which acts as a target) so the player knows where to hit it (the width of the shootable piece will be random to make it harder to shoot). After the piece is hit, the whole tube should get the same color and a next ring gets spawn. I added an example in which I use 2 separate objects (which is not the best solution) as an attachment. I want the tube to be smooth. What would be the best technique for this?

Thanks in advance!