Change background color on a single slide in Microsoft Powerpoint?

The problem that you’re colliding with is the confusion in Microsoft Powerpoint between the “master slide” and the characteristics of a given slide. Pick the wrong one and you’ll find that you’ve potentially ruined your entire presentation, not good.
The way I think about this is that the master slide is the idealized version of a given slide: if you don’t make any tweaks or modifications then your individual slides will all look exactly like the master.
In Powerpoint, your master slide can define colors, typefaces, type treatment, bullets, even background slide images. Everything about the presentation of the material can be specified in the master slide area. The content itself? That you’ll still have to do yourself (though I’m sure that’s something they’re hoping Microsoft Labs will solve in the near future)
Recognizing that different versions of MS Powerpoint are different in interface, I nonetheless proceed with the latest version of Microsoft Office for the Mac, not the windows app… I’ll dig up Office ’07 and add that later.
Start up Powerpoint, load in a slide presentation and when you choose Format from the menu options, here’s what you see:

The key in my experience is not to choose either to tweak the Master Slide or “Slide Color Scheme…” but instead to choose “Slide Background”. Now you’ll see:

That’s where the “apply” button hangs out and with it you should be able to proceed further. If it’s still not working, leave a comment and we’ll work some more on helping you with this annoying problem.