I recently wrote some additions to a nice little utility that changes the background color in forms. For each company you can set the background color to a different color of your liking. The color serves as a visual reminder of which company you are currently logged into. Enjoy!
Addendum: I just added a project that does the same for AX 2012 R2. No additional functionality, just made it compatible with AX 2012 R2. The download is now a zipped file that contains a project for AX 2012, and AX 2012 R2. Please import the correct project for your version..