I think I'd go with brown.
There isn't much interesting in the world thats brown, and those things that are (modern shooters for example) actually look ok with the brown as monochrome.
I figure that theres stuff in all the other colours that I care about, but brown pfft.
Edit -
Just FYI, if you can't see a colour, things of that colour arn't just invisible to you, you just don't see what colour they are. Kudos on the last two guys making the exact same joke... and tbh I suspect we're going to see it re-hased a good dozen or more times by page three...
Basically, its not funny, and it doesn't give you an excuse to do anything.