In case it is not on your mind: currently purple is 1% of players in the ladder and afaik a player is shown as having the best color he has in any of the ladders.
For casual ladder, I see the number of players varying between 2000 and 3000 (at server restarts some people are counted as inactive and hidden, hence the lower active numbers that appear occasionally). This implies that for casual ladder purple is between 20 and 30 people, which is not very far from the proposed 10 and would not leave too many purples either.
For matchmaking ladder, the number of players currently is 200, and I do not follow to know how much it can reach. This implies that for matchmaking ladder purple is 2 people, which would mean that adding green would mean that 1 or 2 players that are high on matchmaking would get a "higher" color than otherwise (looking at the first 10 now, who are not already purple).
For me personally, both effect seem very minor and are not worth the effort of potential confusion / need of explanations / development / etc.
