Hi Bruce, I'm assuming that you're referring to the Colorbox plugin in the Image Gallery feature. I don't know of a way to place the text on top of the image--it would seem to be problematic because the text may disappear if it happens to match the color in that area of your slideshow's photo(s). However, you can always check in with the Colorbox Google Group and see if anyone has a tip for you.
If you come up with a solution, I'd recommend that you clone the Colorbox CSS file to a new name (for example, copy /ClientScript/colorbox/cb_mojo_dark1.css to /ClientScript/colorbox/cb_bruce_dark1.css), make changes to the new copy, and update your skin's style.config to use the new copy. That way, there will be no danger of accidentally overwriting your changes during an upgrade.
I'd be interested to hear about a solution if you find one.