Our oldest children get to be our guinea pigs. With Natalie I tried to teach her colors by only pointing out the colors that were red. That backfired in a big way and it took her a long time to understand there were more color names than red. Ooops. So, with our additional children, I have used color boxes. I take a box, you could use a generic box, and then we walk around the house looking for things that have the color of the day on them. If it does not fit in the box, they can just touch it. Bob Jones PreSchool Material, Footsteps for Fours, has a fun, "we are going on a color hunt," song.
Use this fun way to teach your kids their colors. Your other kids will want in on the action, too. They can either take turns finding orange, or you can let them come and sing along, but let them know it's your other child's school.