Coloring books generally have a theme of sorts. It might be that they are based on a Disney movie, or it is about animals, or vehicles, or something else. When selecting books for your children, remember that they will want to express their creativity, and will have a lot of fun with any coloring book, but will be more interested in a coloring booklet with a theme that appeals to them. Choose a color book that features their favorite characters, or the item of interest in their life. For example, if your little boy just loves trains, get a Thomas the Train coloring book. If your little girl is as girly as they come, look for the Disney Princess coloring book, or possibly Strawberry Shortcake.
This will help them to not only recognize different colors but to also learn the names of the different colors. Developing these skills will help in later development too as your child begins to learn to read and write. He or she can then learn how to write the names of all of the different colors. You see learning is progressive and incremental - it happens in stages. Step-by-step as I have described. This is the true beauty of learning...the more that your child puts in, the more they will get out of learning. As their knowledge grows, so will their interest in a range of different subjects.