Is there a way to change the color or appearance of the mac menu bar from the standard gray?
Unfortunately, there's no setting to change the color of the menu bar itself.
As penguinrob suggests, you could set the menubar to translucent, and influence its color by changing the color of the part of the wallpaper underneath the menubar. LeoColorBar, pictured below, basically automates that for you.
Despite the fact that it hasn't been updated in a while, it's worked perfectly for me in Lion.
A black menu bar
Alternatively, there are applications like Nocturne that you can use to set your menu bar to dark gray.
MenuBarFilter is another application based on Nocturne that turns your menu bar black.
The latter two applications, however, didn't work very well for me in Lion (especially in full-screen mode), so your best bet would be to stick with LeoColorBar.
You can use the free LeoColorBar utility. It's very simple, and seems to work very well: just choose the preferred color, change its darkness, and you're all set. As a bonus, you can change your wallpaper within the app as well, and add arbitrary text to the menu.
In System Preferences -> Desktop & Screen Saver -> Desktop, you can check Translucent Menu Bar, which allows some of your wallpaper to show through.
http://eece.github.com/MenuBarFilter/ allows you to have a black menu bar, although last time I tried it, there were issues with full-screen apps on Lion.