Black pigment in a creature becomes a red hue instead
These "black" areas are still affected by black-altering genes such as merle, vitiligo, smoked.
These areas may look red but are not affected by genes that target red coloration, such as axanthic mutation.
Skin/tongue/etc also loses black pigment if it was present, becoming pink instead
Different black markings may become different shades of red
Erythristic does NOT affect the Sooty gene. Only the Sooty gene is exempt, all other black pigment will turn to red shades. (In this way, Sooty can preserve some black markings by "covering" them)