I really liked the idea of this book, and although occasionally I thought it was stretching (silver for Esther's crown?) I generally think it does a good job of taking the child through the Jewish year (Rosh Hashanah, Yom Kippur, Sukkot, Simchat Torah, Hanukkah, Tu B'Shevat, Purim, Passover -- and then Shabbat) and on each page using a major image associated with that holiday to teach a color.
If your kid isn't Jewish, I wouldn't recommend starting with this book to teach colors, since you'll get sidetracked explaining the different concepts, but once a kid is more solid with colors, this could be a good way to reinforce colors while also introducing them to Jewish traditions.
(Obviously this book is intended for a Jewish audience -- and was published by a Jewish publisher -- and its merit should not be decided by its accessibility to goyim, but my personal experience of this book is as a non-Jewish person.)