Color and Contrast
Color and contrast is how well a color stands out against the colors around it.
Think of it this way
A red strawberry sitting on a white plate is easy to see. The red color stands out because the colors around it are very different. Put that same strawberry on a red tablecloth, and it becomes hard to spot. The more different two colors are, the more contrast they have.
Explaining color and contrast by grade level
Think about a sign on a road. White words on a dark green sign are easy to read. Red words on that same sign are much harder to see. Some colors pop out more when they sit next to other colors.