This is great practice with the not only the colors, but also verb conjugation. The key is based on the Spanish Subject Pronouns (i.e. yo=rojo), and you, the teacher fill in the spaces of the pictures with the different forms of the verb you would like students to practice (i.e. ser, soy, eres...). You can use this activity as an assessment, as a meaningful filler activity the day before the holiday when classes get a little crazy, or even as extra credit.