Check out the USPTO website for more information of types of marks. Generally, marks that are descriptive (as is the case with geogrpahic locations) are less protectable than marks that are simply arbitrary words. This is not to say that your mark is not worthy of registration, but you may be looking at registration on the secondary register. www.arclg.com
This is probably more of a Trade Dress issue, especially if you have laready secured trademark rights. You might want to avoid the burden of trying to defend a Trade Dress or unfair competition claim by chaning the color of the shirt, otherwise you shoudl consult with an attorney on possible strategies for your particular issue.
If you can prove to the USPTO that you have senior user status and legitimate trademark rights to the "saying", you might be able to register the mark. The issue might come down to an analysis of secondary menaning in the marketplace.