matt betea , Aug 03, 2003; 10:23 p.m.
The main way to do this is through the Hue/Saturation panel under Image/Adjust, or more perferably, Layer/New Adjustment Layer. Once you're in that, go through all the color channels and drag the Saturation slider all the way left. Say the butterfly is yellow, desaturate all colors except for yellow.
If that doesn't work 100% for your image, try using one of the selection tools to outline the subject you want in color. Then go to Select/Inverse and then bring up the Hue/Saturation panel again. This time under Master, bring the saturation slider all the way to the left.
The first way should work most of the time. The second method would be for the times when you have a lot of the same colors in the subject as well as the background. Hope that helps.