http://www.thescripts.com/forum/thread199041.html asks the question "Change background/tab colour of tabControl?". I've been working on this problem for 2 days and have come up with a nice, neat, simply way of doing this that takes almost no pain.
See ** Removed link to external code ** for a completely free piece of sample code on what to do.
The source code is for Access 2003 and includes a VB5 ActiveX DLL along with all source code. It should be a fairly trivial process to use this in Access 2007 with VB6. You could even use it just with Access (no VB) with slight modification. The only code that is reliant on VB are the TwipsPerPixelX and TwipsPerPixelY calls, which could be substituted with hard coded values (probably 15 for most monitors).
TabControls in Microsoft Access do not implement the BackColor property and only implement the BackStyle property when the Access MDB file has "Use Themed Controls" property set to false. If you want to be able to have your TabControl use a transparent background, as I do in my code, you MUST set the "Use Windows Themed Controls on Forms" option to unchecked in Options -> Forms/Reports.