With the next generation, should I just select for the best color? And then get a rooster from another line to breed for the next generation?
Keep selecting for the best color. Even as day-old chicks you can generally tell which will be the best buffs. Those with buff down all over will be best. Those with buff bodies, but lighter heads won't be as nice. Until you get all good ones you have to keep using what you have.
I think too often people want to bring in new blood or another line as you say. I suggest keeping at least two breeding pens for every variety you have to develop your own other line. If they are pens A and B, use the cockerels from A over B and those from B over A. This way you won't be breeding siblings or even parent with offspring.
With my bantam buffs I have 4 pens and wish I had tried going with that many years ago.
I didn't accept many orders for LF buffs this year so I could keep more chicks for myself and try to offer more and better buffs next year. I've outcrossed with wheaten a couple/few years ago to make improvements. Years ago I did an outcross with black and gave up on it. I'm doing it again this year. The blacks just have so much going for them that the buffs could benefit from. Of course this outcross isn't a quick fix.
Note the 4 chicks (hatched today) in the photo are from the same buff cock over my best black hen. Three appear whitish while one is mostly black. They all show some red/buff on their faces and have dark shanks. It appears the buff cock carries one copy of dominant white.