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!
At REI, we believe a life outdoors is a life well lived.
@Storm Other good ways to help make them unique and colorful:
Water bottle cages (lots of variety here)
Colored cable kits (Jagwire is a popular brand with many colors, kits are $25-30 each for shifters/brakes. It's an easy job but does require good cable cutters, take to a bike shop for install if you need to. Cables are wear items that need replacing anyway, so it can take the sting away if you do this when the old ones are worn anyways.)
Colored grips (again lots of variety in brands/styles)
"bling" pieces: things like various bolts around the bike, air valve caps, crimped cable end caps, lots of these can be found as anodized aluminum in many colors. These can range from just a couple bucks to $30 for larger pieces like seat post clamps
It can admittedly add up in cost if you do a lot of these, but they undoubtedly make your bike yours and yours alone!