Can poinsettias be yellow

Yes, poinsettias (Euphorbia pulcherrima) can be blue.

Specifically, this achievement reflects the scientific and technological changes of the 20th and 21st centuries. Among the possibilities that such changes offer is the ability to access a plant's internal structure and affect its biochemistry. For example, red, purple or blue colors are due to the presence of pigments called anthocyanins. Other pigments include the chlorophylls responsible for green colors and the carotenoids responsible for ivory, orange and yellow colors.