A great little kids book is Raindrops: A Shower of Colors. This book is a great way to reinforce colors to your children. Each page has a raindrop of a different color on it. This book also shows how primary colors magically blend together to create secondary ones through the raindrops. Even though it's a very simple book, both my kids really enjoy it.
The graphics are beyond cute, and the board book is of the perfect length to keep the shortest attention span interested. This is a must get book for the younger set!