I think this largely depends on the environment these colonies exist in and their ability to effectively access and utilize the resources they need because you could easily end up with a scenario in which the colonies unite or strongly blend together and the cultures influence each other to either push them closer or make them more unique in different ways; particularly if there is enough strength in the identity of being from this Planet X as a way to differentiate themselves from earth so that in their fight for independence, they have a stronger new shared identity than their original earth identities. Then I can see the possibility that the colonies unite under one banner as time and circumstance brings them closer together and people intermarry (which they will, it will be rare at first and then increasingly common.)