What impact, if any, does the solder mask color have on the electrical and mechanical properties of a PCB
I'm certainly no expert, but the other day I came across this article from a PCB manufacturer that I would like to bring to your attention. It states:


  
  Green solder mask is physically superior
  
  
  Due to the traditional aspect of using green pigments, R&amp;D into making better solder mask oils focused on the standard green color. The actual chemical pigments used influence the performance of the resin during application and in the field. When demand is pushing form factors to their limits, no one cares about having pink boards. As a result, green solder mask now has superior properties compared to other aesthetically orientated colors.
  
  Most prominently, commercial green solder mask is the only available color that can reliably produce solder mask dams of 0.1mm (4mil). Next up is red, yellow and blue that can produce 0.12mm dams and then black and white which can only achieve 0.15mm. Solder mask dams are vital for ICs and fine pitch components since they are valuable in preventing solder bridges from forming.
  
  


So yes, based upon this information I would say that there are differences in quality, as the factory itself states. Green would be the way to go for best quality.

Hope this helps.

Edit: Added picture of solder mask dam, from here