I used to have pet rats and mice, but I have no issues with someone keeping a breeding colony or program to feed their snakes. For me, my reluctance to own snakes stems solely from my concerns about being able to feed them. Yes, you can buy pre-killed, frozen rats and mice in all sizes easily enough and I have no qualms about feeding them, but I do worry that the pre-killed rodents were not killed humanely. I would never feed live prey as I think that can be cruel to the rodents and can be potentially dangerous for the snake as well, but I am not currently confident in my ability to humanely kill a rat or mouse.
1. Like with keeping chickens I suppose this would depend a lot on the local laws in your city/county/state. I doubt there would be an enforceable permit for raising a few rodents for your own personal consumption. If you were raising a large number and selling them to pet stores or individual snake owners there might be.
2. It's been a long time since I've looked at the prices of feeder rats, but I want to say a med-large rat was around $5, less for smaller rats, mice, and pinkies, and more for large rats.
3. The most economical, and nutritiously complete, feeds are going to be lab blocks. Rodents and their nutritional requirements have been so closely studied in laboratories for so long that I would argue no other domestic animal in the world has a more nutritionally complete commercial diet available. Harlan-Teklad and Mazuri are the only two brands I am familiar with, but I'm sure there are more. And anywhere that can order you chicken feed should be able to easily order lab blocks for you.
4. Snakes don't care one way or the other what color the rodent is. Their prey drive is based more on smell...so if a snake owner is having a hard time getting a snake to eat a mouse or rat it is often recommended that they rub the rodent on a prey species that the snake would prey on naturally in the wild to entice them to eat. White and brown agouti are the most common colors for feeder rats but probably just because they all kind of look the same and the people feeding the snakes are less likely to get attached or feel sorry for them. Black hooded rats are also pretty common, although they tend to have patterns that are somewhat unique from individual to individual. For someone with an interest in color genetics, culling the extras or rats/mice that don't quite meet the color standard to which they are trying to breed as snake food can be an ethical practice (although many pet rat/mouse owners/breeders don't see it that way).