Rainbow gets you to 7, which I can do without thinking: red, orange, yellow, green, blue, indigo, violet. 2 biggest non rainbow colours: pink, brown. Then you just need to name any of the more unique colours: teal, cyan. That’s where I got to before I had to actually stop and think. You could probably add another to the rainbow by listing purple as distinct from violet. You should probably avoid black and white in case the person is being pedantic.
He was born in 89 which is very close to my age. Most people my age that are kind of techy but don’t do it as a job or main hobby probably tried Linux around 2010 or so, at which time it was rubbish for gaming, and wrote it off as impractical. It absolutely makes sense for someone that hasn’t tried it since then to say “now” when referring to its ability to play most games well, because it’s relative to when they last tried it.