If you have the 2.2 beta active, you could mod it. You'll have to go to your "My Documents/Longbow Digital Arts/Hegemony Rome/Mods" folder (if it doesn't exist, create it), add a new folder for your mod, add a Resources folder in your mod's folder, and create a .xnt file in there. The 2.2 beta comes with example mods you can look at in the Mods folder found in the game folder (not the My Documents) if you're confused.

The contents of the mod you'll want should look something like:
<entity modtechnique="merge" class="faction" type="CisalpineGaul">
<guicolour>#ffffff</guicolour>
<unitcolour>#ffffff</unitcolour>
</entity>This will change the Province's faction colour to pure, vivid white. You probably want a different colour: the colour is in the format "rrggbb", where "rr" can be 00 for zero red or ff for as much red as possible. You should be able to find hexadecimal colour preview things on the internet to help you come up with a colour you want.

Note that this won't change the icon or flag to one with matching colours. You could add
<flag atlas="FILE:" sprite="Resources/Images/somenewflag.dds"/>
<logo atlas="FILE:" sprite="Resources/Images/somenewlogo.dds"/>
which will change the flag and logo to use your images if you make ones that match your new colour.