When viewing your roster, there is a banner on the top left that says “Roster”. I never really paid attention to it, but last week I sold my 2* Thor, my total roster went from 90 to 89, and the color of that banner changed from red to yellow. I then re-rostered him, putting my roster back at 90, and then the banner went back to red. I know it has no game impact, but is there any meaning or reason to this?
The meaning would appear to be that red signifies you have no more open slots… the reason would be that someone on the dev team has a really unhealthy obsession with banners…
Good thing they have that handy feature programmed in…otherwise, how would I know if I have an open roster slot or not?..
green = 2+ free, yellow = 1 free, red = buy Hero Points so you can roster that character who just dropped
Mine is always red, because I stick to the limit!!! Why would I ever want unused roster slots though? So I can have duplicate Yelenas? 
It’s been a while since I’ve had 2 free slots, so the ceiling between yellow and green might be higher, I don’t know. Maybe it’s 1-3 yellow, 4+ green.
Either way, green is like ‘you got lotsa slots you’re good,’ yellow is ‘danger danger will robinson’ and red is ‘insert coin to continue’