1. Type a word you want flagged (make sure it is all lowercase).
2. Apply Highlight or text shading or a font color to the word.
3. Select the word.
4. Go to File | Options | Proofing: AutoCorrection Options.
5. In the AutoCorrect dialog, you will see the selected word in the "With" box.
6. Click the "Formatted" radio button, and you'll now see the word with the highlight, shading, or font color.
7. In the "Replace" box, type or paste the same word.
8. The next time you type this word, it will have the formatting you applied.
PPS: In Word 2010 it can be a bit of a challenge at first to get beyond step 4 but using the F7- / "Options" / Auto Correct Options/ I did manage