It's the new design since Lion - Iphoto already has gone gray before Aperture See these links:
How to bring back the color: (for experienced users only)
Re: Have today loaded the 3.3 up date on Aperture. After the up date the colour of the Menu Bar and all icons showing projects / albums etc are now grey. Any thoughts on how to reestablish the colours.
"Why" is because Aperture is a professional image management application. I agree that the ability to switch between color/monochrome would be very desirable.
Color anywhere on the display(s) affects the color perception of images on the display(s). I for one think that for a professional image management application available monochrome for the app is much preferable, even mandatory.
Personally I strongly congratulate the Aperture team for the move to monochrome. I suggest that folks who like to have color on the app itself request an option to apply color to the app. Best would be a color on/off toggle so we could have a professional monochrome workspace for editing but switch to color for organizing.
Just my 02.