I have 4 teenage boys and I used to dress them in bright stripes and tights when they were little (in my defense, tights on boys are normal where I come from). I love bright colours and they all looked adorable etc. When they went to school I stuck to the standard boys uniform of grey shorts all year round. The grey shorts are no big deal, 100% cotton from M&S and you size up so they hang down below the knees. Still looked adorable I have to say, but not as colourful, of course. However, there are weekends and evenings and non-uniform days and this was in the days of neon football boots so there is still plenty of opportunity for colour outside school.
But you're right, they then all enter the teenage 'everything mucs be black, grey or, at a push, navy' phase, which is a little depressing. But I wanted to sound a note of optimism, the colourful football shirts and boots often do prevail and they do eventually exert their own sense of style and, perhaps, early colour brain-washing from their mothers. I am just watching my eldest walk up the drive, back from uni, and he is wearing one of the most day-glo pink Nike training tops I have ever seen, orange shorts (it is raining) and bright white crocs. Be careful what you wish for I guess😆