I have officially hated bumpers since our first season (we are a fifth year team).
And it really isn't the bumpers per se but the mounting of the bumpers that I hate.
Anyway, bumper materials, colors and the numbers are something that should be inconsequential and rote purchases for the robot. We have always used Duck Canvas (Royal Blue and Red) purchased from pretty much any local fabric store. Duck Canvas is tough yet workable. Both colors closely match FIRST's colors and are nice vivid colors. We take them to the school's screen printer (any screen printer can do it) and have them put bright white letters that conform to the rules centered on each piece of the bumper. This has worked well for us for four years.
Our best bumpers were last year when we use the reversible bumpers. They are so wonderful and so quick to change that it just reduces the stress of quick changes for everyone.
Mounting the bumpers in a secure manner is always our challenge.
My recommendation is to not make the bumper colors, material and numbers something you are going to be stressing out about at the competitions. Keep them simple. Promote your team image on the robot itself, not the bumpers.