Red-green is the most common form of colour-blindness, I think it affects something like one in twenty Caucasian men, and it seems reasonable to assume that Caucasian men will make up a significant fraction of the users of any random web-based thing, I've had to make this point to several groups I've worked with in the past. The database curator at my previous place of employment was red-green colour-blind and was helpful about things that were not clearly distinguished to him. Yellow-blue's a lot less common, but I think it is the next most common form, and I'm not aware of any others save for lacking in colour vision altogether. I think you've probably got a good enough brightness distinction between the two categories, but what you don't have is an indication of whether the brighter or darker is yellow for people to whom that's not visible.