IMO 5 weeks is way too early to cull buff roosters. They change so much over the next several months, you could very easily give up an excellent specimen. This is especially important if you are planning to breed or show. I know many who culled them all and only kept one and that one ended up growing out poorly. You can easily shoot yourself in the foot making rash decisions.
As far as early things to look for..yellow is a definite no-no (beak or legs). Carefully inspect legs for random feather sprigs. It is not uncommon to get one now and then and between 5-10 weeks the scale lifting will occur indicating a feather is coming. The only color I would cull for is white, especially a lot of white down or any signs of a completely white feather in the plumage. Any obvious defects should be culled right away. Toes problems, beak problems, side sprigs on the comb, wavey or floppy comb, etc.
I would not cull any showing sign of black speckling in the plumage (esp the tail) until they mature more. Buff and golden color will change a lot as well as they mature. You are looking for an even distribution of buff, but that won't be fully noticeable until almost a year old due to the various moults they go through. You can watch for long legged birds and keep ones with shorter to medium length legs. Combs should stand erect and look like a blade from head on view, with 5 single points. Again, most of this won't be as obvious this early, but hopefully it gives you some idea of things to look for.
Jody
