The latter - black whitish grey, how to I fix it. It did it as soon as I downloaded it, first time I ever used it. Thanks.
Urgh. I was afraid you'd ask me that. The first thing I'd try is to re-download the program, perhaps from a different site if there's a choice. But assuming that doesn't help:
I'm a Unix/Linux expert, not a Mac expert. However, Macs run a variant of Unix under the covers. So let's give this a try:
In the Finder window, click on Applications on the left, then scroll down to Utilities and open that folder. Inside Utilities, near the bottom, you'll find Terminal. Launch that. You'll get a small white window with black text.
Click on the Terminal window so that you can type in it, and type this:
and hit the Enter key. What should happen is that Gimp launches properly. But what will actually happen is that you'll get the same behavior you've already seen, but a bunch of messages will appear in the Terminal window. Cut and paste those messages into a posting here, and I'll try to puzzle it out.