Welcome to the shrimp keeping hobby!
Neocaridinas breed readily with other neocaridinas. You certainly could buy some blue neos and put them in with your red neos, but then you'd potentially revert to wild type coloration in their offspring. You definitely will not get an additive color effect (like blue + red = purple... nope nope nope). Neos don't breed that way. If you're lucky and the blue neos you get were bred from a red line cherry shrimp line, you may get mixtures of blue and red. Maybe. Probably not. Chances are you will get wild type coloration. But the adults you buy will keep their colors as long as they're alive. If you like the splash of color, and don't mind the offspring, hey go for it.
There are other shrimp species that can coexist with your neos, like caridina cantonensis "Tiger Shrimp", or caridina serrata "tangerine tiger shrimp", to name some that come to mind. They're readily available online. These shrimp I've mentioned are more difficult to keep alive, so maybe make sure your red cherry shrimp do well for a while before making the investment into caridinas.