My son is 12 and has been playing trombone for 4 years with a student model. He has become a trombonist of a youth symphonic band and is looking for a trombone with F attachment that will last till college.
How do I compare between the quality of different brands, such as Bach 42T 42AF, Getzen 3047AF or Edwards, in terms of tone quality and playing skill needed? Is axial flow valve an appropriate choice? How to choose between yellow brass, rose brass and gold brass? I'm not limited by budget as long as it is worth the money.

I want to tell you first hand: All three of those brands are GREAT brands for trombones. But if you want your son to have the trombone until college, I would go with the Getzen. Getzen trombones are world-renown for their DURABILITY.

I just got a Bach 42BO after playing trombone for 4 years.
I tried out several horns: Bach 42BO, Conn 88H, and Yamaha YSL-620
I previously played a Yamaha YSL-354.
I recommend those brands, but in the end its all up to your sons preference, such as weight, wrap (open or closed), bore etc.
I am so happy with my Bach and would not change horns. also, most pros use either edwards, shires, greenhoe, rath, yamaha, bach, or conn. good luck with the search

All these brands are excellent.

I can tell you my recommendation but the reality is that every player is different and that means every player will need different equipment.

What I have found teaching privately is that students tend to use better air support with an open wrap, but again this is a very general observation. Another observation is that students have a slightly brighter sound (more cut in the sound) if they use a yellow brass bell and that a rose brass bell sometimes yields a darker, richer sound.

Some may disagree with me, but I personally don't think a beginner necessarily needs a world class custom Edwards, Shires, or Greenhoe to learn on. These are all top of the line horns that I've always seen people buy in the latter years of college and even late into their careers. That's just my opinion.

I got my first professional trombone when I was 12, and it was a Conn 88H, open wrap (rotary valve) with a rose brass bell and let me tell you that horn is amazing. It lasted me until my junior year of college, and would've gone past that if I hadn't discovered a Shires, which is my favorite trombone I've ever played by far.

I would recommend a Conn 88H or a Bach Strad. for them to learn on, then if they continue in college, an Edwards or Shires.