In Photoshop CS2, you can use the cool Colour Replacement tool to swap out colours as you work. Choose the Colour Replacement tool from the Toolbox. Go to the tool options bar and set the Mode pop-up menu to Colour, the Sampling pop-up menu to Once, and the Limits pop-up menu to Find Edges. Now Alt+click on the colour you want to replace. Click on the Set Foreground Colour swatch in the Toolbox and choose a new colour to use in your image. Now, just paint away the old colour.