How do I change the colors used in MS Word 2007 track changes?

Apparently you can't, see "Change the way that markup is displayed". Microsoft says:

you can't assign specific colors for changes made by different reviewers

You just go to 'Change Tracking Options', and choose your color. There isn't a large variety, but it's better than red.

Insertions are marked in red with underlining, deletions are marked in red with strikethrough, while moved text is marked in green with double strikethrough and underlining. Word also places a vertical line in the margin next to any changed lines. The vertical lines and other revision marks may be printed.

To view the track changes default markup and color:

On the Review tab, in the Tracking group, click the bottom half of the Track Changes tool and select Change Tracking Options. The Track Changes Options window opens.

If you prefer that all of your edits be in one color, such as red, you can uncheck the Track moves checkbox in the Moves area so that the green formatting is turned off and all of your changes will be in red.

Later you will see that Word assigns a different color to each reviewer of the document.

From here (pdf link), on page 9. It's very useful!