I don't breed mine yet (just have 3.3 as pets right now), but I wanted to mention a couple things to consider.
1. You will need to decide how you will dispatch them first (Co2 chamber, spinal dislocation, etc.).. You will end up with more than your snakes can eat, so more than likely will need to kill / freeze or sell your extras.
2. They smell... a lot... do you have access to an area to keep them with ventilation, but at good temps (this is the number one reason I'm on the fence about breeding myself)?

For your questions. I think the main thing with picking them out is to get healthy and friendly stock. See if there are any feeder breeders in your area and they may have rats available (check FB for local groups). I would definitely go with ones bred for temperament as getting bit is no fun. If you have to go with pet store or expo stock, watch out for sneezing and signs of ill health (patchy coats, dark fluid from the nose / eyes, etc.).

I don't think colors matter, but if you want to be able to sell extras off as "pets" you might want to consider getting a range of colors / markings... and the "dumbo" eared ones are always popular as well.

Good luck!