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.