Does this sound fair?

I would say 'be very careful'.

You are focusing on the colour, but experience has shown me that colour is not the main concern, the quality of the painting is much more of an issue. There is a massive difference between a wall painted by a professional and a complete slob who won't bother putting masking tape and will end up painting on the ceiling, skirting boards, fire places etc... And of course, even if they agree to re-paint when they go if you don't agree to the colour, you can expect it to be an even worse paint job.

Unless your place looks a bit tatty and any paint job is better than current and it doesn't matter if you are left with some blue paint on the the white ceiling, then go ahead, otherwise, you are much better agreeing to paying half and getting a decorator doing it (or them paying a decorator in full themselves if it has just been decorated already).

The argument is that if they do a terrible job, you can claim some of their deposit, but the reality is that arbitrators are never very sympathetic towards LL when it comes to redecorating costs and to be fair, pictures rarely pick up how bad in can be in reality (such as a wall being only painted with one coat or very cheap paint).