Hi, iantommycampbell

Assuming the t-shirt is the only element in your clipart, try selecting it on your page to make the "Format" tab visible. Then click "Color" in the "Adjust" group and select the color you'd like to apply under "Recolor." Sorry, I'm using PowerPoint 2010, but I believe the steps are similar in PowerPoint 2007.

If your image includes other elements, you can use Pixlr, a free online editor to upload, edit and save the image. Their direct link is: http://pixlr.com/editor/

Hope this helps.