how do you deny someone else their colour？ i heard it was membership and level, but a level 20 player non member senied my colour.

To deny someone their colour, you need to join a game before someone with the same colour as you does. However, if both of you are invited, the player with the most points gets their primary colour.

Membership doesn’t do any difference to colour. Where did you hear that it did?

It gives Color Superiority achievement.
Example:
Onigr5 and I get invited.
Both of us join, but since both are Electric Purple, the one with a higher level gets their primary (me), and the lower level, if the alt color also gets taken, an alternative color is used solely for that game. If it isn't, the alt color gets used.