I find it amazing, indeed, but if I look in the help for "backcolor" (that's the general property for background color) and click "applies to" Access tells me that it applies to a lot of things, but not to buttons.
In fact, executing the code Me!button0.backcolor = 0 does give an error message that this propery or method isn't supported by this object.

This is Access 97, but it's easily tried in any other version, of course.

There must be a reason for this lack of flexibility, but it's a little bit strange. Luckily, there is a workaround. You can make a bitmap in any color with anything on it you like in any tool (MS Paint will do) and use that as the picture to be displayed on the button (choose the ... and browse to the picture). I've never tried it, but I assume it will work satisfactory.