Consider the following opinions of what adumah can or does in fact mean.
Sadia gaon writes that Aduma of the Parah Adumah is 'Tzafra'. The footnote from Rav Kapach in the Mosad Rav Cook edition explains this means yellow, like an egg yolk.
Torah Temimah on the first passuk in chukas #8 says Adumah could have meant black, if not for the fact that the Torah specifies temimah.
Does anyone else suggest any other colors adumah might mean?