[@Majoras End] The red coin just changes your magic, without really adding anything. Think of a magical girl as a glass, and the coins as different types of liquid.

bronze/silver/gold/plat liquid adds magic to the glass.

Black liquid corrupts the magic in the glass, and adds a sizable chunk to it.

White liquid causes the corrupted magic to evaporate

Red liquid causes the magic to change color, but does not corrupt or add anything to it.

A red coin doesn't have a violent reaction because of what it does. The bates took a white and red coin at the same time and it just had the effects of both. But a black and white coin directly oppose each other and cause violent change inside a magical girl, which has deadly consequences. 

Or something like that.