erythrism

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Unusual red pigmentation, as of hair or plumage.

from The Century Dictionary.

• noun In ornithology, a condition of dichromatism characterized by excess of red pigment in the plumage of birds which are normally brown, gray, etc.
• noun In anthropology, redness of hair, generally combined with light complexion.
• noun In zoology, the assumption of red in animals, in place of their ordinary coloration.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) A condition of excessive redness. See erythrochroism.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biology Abnormal red colouration.