bitwisenot asked: Would Phyrexian mana have worked better if it was used only in the generic part of the mana costs? (So no colored phyrexian mana costs) That way colored mana would still be required for effects, helping protect the color pie?

The color pie problems were due to just a few cards. We should have simply avoided those handful of effects. (Which Design did, by the way.)

Changing the mana from colored to generic would have greatly lessened the utility and flexibility of the mechanic.