Aperture's levels control can be used to easily add color to otherwise gray images. This original photo of stacked chairs has a range of grays that exist only on the chair backs and seats:

By selecting the Blue channel on the levels control, I can alter the amount of blue in the gray part of the chairs. In this case I'll boost the darker tones significantly to give a blue tint:

With some boosting of the saturation with the exposure control and a crop to focus attention where I want it, I get my final image:

Of course the colors can be cut or boosted individually or in combination to get any color. The Colors control can also be used to adjust the color once one is applied.