Answer:
Green and red are the original colors of the bracts of poinsettias (Euphorbia pulcherrima).
Specifically, bracts are modified leaves. Poinsettia bracts may be true or transitional. True bracts immediately surround the stem tips where the dainty, inconspicuous yellow flowers appear in December. They are always red.
Transitional bracts are located between the true bracts and the plant's regular, green leaves. They start out green, but change color in sync with the initiation, development and culmination of the plant's flowering process. Red is the traditional color that transitional bracts become. But with the scientific and technological changes of the 20th and 21st centuries, many other colors - such as blue and yellow - are possible with access to the plant's internal structure and biochemistry.