The youngest children often come to school knowing the names of many colors or very few. Montessori color boxes are used to introduce colors to the children. A variety of activities and songs are also a fun way to help children learn colors. Hap Palmer and Frank Leto both have great songs about colors, which can be easily be found online.
Children are excited to discover that some colors are called primary ones and others are called secondary colors. An activity to mix primary colors and watch them change to secondary colors is a very common, poplar, and fun exercise for all the children, including the younger ones. Older children enjoy learning about the color wheel and identifying the color names on the wheel. I often have the color mixing activity set up on a particular table in the classroom, if possible.