It already exists: https://warlight.uservoice.com/forums/77051-warlight-features/suggestions/2382010-secondary-colors
Also, I'm not sure it would solve the problem you're facing most of the time.
Think about it this way: If we assume that the other player who took the last spot didn't get his color as well (which is true 95% of the time in a big FFA), and assuming that you both pick your color at about the same speed, then he'd still get that last spot before you even if you had this feature.
This is because he'd be able to skip the color selection screen too. And if he clicked Join Game before you, he'd get the last spot.