Hi
I am 90% sure this cannot be specifically done in Aperture as it can in Photoshop.
I assume you mean adding spot colour to a B&W photograph.
You can choose an external editor such as Photoshop, if you have it, create the effect, and re-import the image back into Aperture.
There are some powerful colour controls for selectively improving colour in a photo within Aperture. Whereby you can make a particular colour stand out more by selectively adjusting the red, blue,cyan,magenta, yellow colour's. These are accessed via the colour controls in the adjustments inspector.
You can create B&W images with two tones (of your choice) by using the Monochrome mixer ( in the adjustments panel )
Or you can create colour monochrome effects by using the colour monochrome effects slider ( in the adjustments panel, where you see a + sign with a downward pointing arrow ) click the arrow and select "colour monochrome" from the menu, it will appear at the bottom of the adjustments panel.
Give some of these a go..... play around with it, you can get some stunning result's..........
Hope this helps some.......
w..............