That is the new Look & Feel, to fit in with the overall design of Lion and Mt. Lion. Some like it, some hate it. I find it really hard to recognize the different buttons at a glance without the aid of the color and colored my icons again.
If you really hate the grey design, you can bring back the color by replacing the images of the buttons inside the Aperture application package. This is a risky fix - do it only, if you are an experienced user. And you will have to do it over and over again, with each software upgrade
I was wondering that as well, personally I think it's horrible and they should offer the option to let you choose, like yourself I find it hard to recognise at a glance....
Use the Aperture feedback option and have a moan..... I just did.