We told our kids that there are many types of vehicles. And many of them have different numbers of wheels. So we tried to find out this difference. 

We gave them worksheets from a book with different motor vehicles and ask them to paint different vehicles in specific colors depending on numbers of wheels they have. 

In that way we memorize all of those cars and make a revision of colors we know also it helps to make an association between color

 number of wheels and the name of a vehicle, which makes it easier to remember.