Aperture makes it very easy to convert a color photograph into a black & white image. Open the Adjustments Inspector and choose the Monochrome Mixer (Control-M). Open the disclosure triangle. You can use the presets or make and mix your own adjustments.
The presets offer you the choice between traditional B&W photo filters such as red, orange, yellow, green or blue. If you want to make a custom filter, here’s the trick. Make sure that the total of the sum of all three sliders (Red, Green, Blue) don’t exceed 100. If you exceed 100, this will probably result in an unnatural or overexposed image. Likewise, a number well below 100 will probably result in a severely underexposed image. Stick close to 100 for the most pleasing results.
NOTE: There is one exception to this rule. If you want to achieve a simulated infrared look to your image, exceed 100.