@Storm Thanks for the question!
The Co-op Cycles REV CTY bike does not come in different colors. However, the REV CTY Kid's bikes do come in two different frame styles: REV CTY and the REV CTY Step-Through that are different colors. One is a step-through style with a little bit lower top tube and the other is a 'regular' style bike frame. Other than that (and the color) the bikes are identical.
One other option to differentiate the bikes for each of your kids would be to get a bike bag like the Revelate Designs Mountain Feedbag Handlebar Bag that comes in multiple colors. My kids love having them on their bikes as they can hold a water bottle or snacks while they are riding.
Hopefully this helps!