What red product did the Aztecs make from poinsettia leaves?

A reddish-purple dye is what the Aztecs of ancient Mexico made from the bracts of poinsettias (Euphorbia pulcherrima).

Specifically, the bracts are modified leaves. There are two kinds. One are the true bracts that spend their entire lives red in color. They immediately surround the plant's dainty, inconspicuous yellow flower. The transitional bracts are between the true bracts and the plant's regular leaves. The transitional bracts change in color from green to red, in time for the Christmas holidays. The regular leaves always stay green in color.