If you color-code the text in your Word documents, you may feel constrained by the 15 Highlight Color choices:
Fortunately, there's another command available if you want to expand your color palette: Shading. The button is not far from the Highlight Color selector on the Word toolbar: It's just about seven spots to the right over in the Paragraph section:
You can use More Colors to select from the expanded Colors dialog:
Or even enter custom RGB values if you like:
This feature is available in both Word 2007 and Word 2010.
A couple of small cautions: If you use my tip for hiding highlighter marks, any text with Shading will still be displayed and printed. Also, you'll have to remove Shading separately from Highlighting - simply select your target text and choose No Color for each.