I've seen this with the Google toolbar. If configured, the latest version of the google toolbar will automatically fill forms with data. By default, the toolbar colors text boxes which it thinks it can auto-fill. I found this "feature" to be particularly annoying and turned it off.
I'm not suggesting that this is exactly what your problem is because the google toolbar colors text boxes yellow, not blue. But, since this seems to be isolated to one computer and IE only, I would check to see if you have some other plugin or toolbar that might be coloring the textbox.
OR: Google determines which fields it can auto-fill by the name of the control. You could change the name of the textbox and see if it still turns colors. If you change the name to something arbitrary and it isn't colored anymore, then it's probably something specific to your browser.
-Ron
We all play from the same deck of cards, it's how we play the hand we are dealt which makes us who we are. -Me