For a RED day, the child could wear a red shirt or red shorts.
When eating point out all the RED foods, watermelon, apple.
Do RED activities, paintings with an emphasis on RED, try making RED playdough. Pick out the RED toys.
Add RED liquid bubble bath to their evening bath.
Throughout the day point out everything that is RED. Soon your child will follow your example, and look for the RED colors in their day.
Be innovative, find other items that are RED, there are lots around the house. A fun way for children to learn colors.
Follow other days with other colors. Learning should be fun for kids, help them to enjoy learning colors.