The yanks have a different way of spelling it, that actually makes sense...
'diarrhea'
All the letters sound out and the redundant 'o's been taken out.
I'm proud of our language and wouldn't want to yank-ify it, but as I have problems spelling the botty problem disease, then I'll let them have this one. It's a definite no to 'color' though. 'Cos that looks wrong more than anything else.
On a side note, my wife is a terrible speller, she had trouble with councillor and rythym last night... the unfortunately ironic thing is that she's a primary school teacher and has to propagate spelling to a reception class.
Thankfully they're as bad as she is, so no-one really notices!