While there are 40 possible slots for custom scenario, I have noticed that there are in total 50 colours. Why have colours AN-AW not been used? I'm sure it would be beneficial if there were more possible colours.

It would be more beneficial if there were more maximum slots possible in a game. The problem is that it's probably "not on our immediate roadmap"

I think Fizzer made a 50 player game at some point

Yeah. If you do custom scenario, Slots AN to AW (AX is smoky black) are colours that you can't play as. Maybe after level 53 you keep gaining since there isn't a big problem.It would be useful for the Issander Diplo I was working on.

Edited 8/12/2016 16:54:14

It doesn't exactly take a lot of work to increase maximum game size. I'm pretty sure Fizzer's holding off on it for other reasons, like server costs and abuse potential.

(I typed color without the "u" on purpose)

Oh sh*t it's getting tactical up in here.