I've had several RC treatments, so can confirm what franchie says (although I doubt I need to). All of mine are front teeth, but I can tell you they have discoloured to a greater or lesser extent over the years. In one of my older crowns, you can actually see the discolouration through the surface of the crown if you look closely.
Whether it's painful or not will depend on you, your dentist and the anaesthetic. My teeth/gums are quite sensitive and I'm a bit of wimp, so my dentist always uses the highest dose of anaesthetic that she's able to. She happens also to be a very careful, accurate and intuitive worker and has only ever once really hurt me beyond the initial injection. That was when she brushed her hand against something holding something in place whilst she was working. I think she must have caught a nerve ending or something and she couldn't apologise enough.
Generally, though, it's not a painful process. You may be a bit sore afterwards, when the anaesthetic wears off, because it's quite traumatic for your poor little gums, but a few paracetamol and some tlc will soon take care of that.