SF's color scheme is subtle. I've posted answers about that before, though I'm having trouble identifying those questions.
<- Server Fault 
<- Security
To the colorblind the top two are nigh indistinguishable (there is an android app that provides colorblind simulation for those with good color vision), with very slight contrast differences. To me the bottom two have good contrast, so long as I'm using a good monitor.
Because I spend a lot of time here (moderation doesn't apply to moderators it seems) I have no trouble seeing the contrast from the bottom two at a glance. Even though I can't hunt up some of my older screen-caps, it does seem to me that this is better than what we started with. Even so, it still isn't the "accepted-is-filled" convention that many other sites follow.
IT Security
That one is also subtle, but the nice minty green fill does provide a different visual cue than we have on SF. Is this better? Maybe. DBA, Unix, and Webmasters all follow a similar minty-green scheme.
SF's scheme predates the rest of StackExchange, so we have that whole 'legacy' thing going for us. The Red/Green thing is an issue that I believe needs fixing. An in-fill color would fix that, in my opinion.