White is a great all-around choice for working on marked retrieves or any drill, really. From piles to marks to blinds, a white bumper has its place. For building confidence on blinds, use a white bumper. Make sure your dog can see it! For marked retrieves, use a white bumper. It will help your dog be as successful as it can be.
Orange is excellent for blind retrieves. Don't be surprised if your dog runs right past an orange bumper. It's harder for your dog to see and, therefore, should be used in the proper context considering variables like wind, cover, etc. Don't be afraid to tape a duck wing or add scent to your bumper to help your dog on the blind retrieve. They have an incredible nose for a reason.
Yellow is an excellent blend of white and orange, in our opinion. In some cases, it's easier for the dog to see than white; however, it all depends on context. The science out there shows that dogs can see yellow well, but in our experience, it all boils down to contrast of the environment. You can use yellow for both marked and blind retrieves. I use them as a good in-between bumper for yellow and orange.