In reply to heist182:
Another colour blind climber here.
As has been said:
Confirm the different colours before you start.
Try to differentiate using texture or style of holds.
Also:
Get a patient belayer.
Try not to get fed up of people taking the p1ss. Honestly, some of them *really* think that "That's a good excuse, I must remember to try that myself" or "Yeah, I say that too when I can't do the route" are original and funny.
It's worth chatting to your local wall staff. I've found you get mixed reactions though.
One wall I used seemed very interested and even sent me an email - I replied but have heard nothing since. Perhaps they decided it wasn't a big enough problem to devote time to? The more people that mention it to them, the more they are likely to see it as a widespread issue, that is worth their time to resolve though.
However, at another wall, I asked if they could maybe think about trying to avoid using purple and blue holds of the same type on the same panel (explaining why). The managers response was "Well, you can ask!".
Great PR there.
At the end of the day, there's not much else you can do. Different people can get confused by different colour combinations. Chalk & rubber on the holds, poor lighting, differing colurs of backgrounds can all make it much worse too.
The best walls I've been to are ones that use plain colours on the background and clean the holds regularly.
Si