These synthetic crystals served just as well, the only difference is they couldn't make any other color than red. Hence, the red lightsabers.
Incorrect. The can easily be made in other colors (for example, Luke's green saber uses a synthetic crystal). However, if steps to create a certain color were not taken, they tended to naturally have a red coloration. It was laziness rather than inability that caused the rash of red sabers.