Neither the Amiga nor Atari held up well when you actually did technical due diligence when it came to supporting real shipping software. I did this for both platforms in 1988. For porting shipping products from MacOS. For the biggest consumer software company at the time. GEM was a godawful mess (but better than Win 2.0) and AmigaOS was a mishmash more like a game consoles hardware libraries than a PC OS.
As for colour. The original 64K ROM Quickdraw supported colour. In 1984. There were companies in both the UK and US who modified 128K/512K Macs adding an external colour monitor. I saw them demo'ed at trade shows in London and San Francisco in 1985. And by the end of 1986 I was using a Mac II with 2 colour monitors. When we got a bunch more monitors a few months later we found it had no problems with 4 monitors on the same machine. One contiguous desktop. In theory we could have added up to 6. But as monitors started at around $1k back then so not exactly plentiful around the office so never got to 6.