Just off the top of my head... so correct me if I'm wrong:
Post-colonial texts are those written by cultures and people who were previously colonised by a foreign power. Generally, their aim is to reintroduce traditional values, ideas, thoughts etc into that country's literature. Usually, it's done through incorporating many of the myths and beliefs of the culture but also by meshing English with the native language to create something that is not at all like a traditional English text and often they have a very different stucture and way of writing. Often, they seek to reconcile the differences between the natives who have been dispossesed by their foreign colonisers and the colonisers so they can live together. It's not reactionary in any sense against the "invaders" but it tries to reestablish a sense of pride and "nationalism/patriotism" amongst the native cultures.
Postmodernism is a way of thought. Many of its ways of thinking can be traced back to early 20th century philosophers such as Nietschze and one of it's key beliefs is "there are no facts, only interpretations". Post-modernism also challenges everything. It doesn't believe there's a traditional anything, it challenges the notions of the modernist period (the modernist period was characterised by a belief in the "new" only and it also believed in grand narratives/metanarratives [an overriding view of the world or sketches of reality in narrative form e.g. Capitalism and Marxism which both espoused that they would emancipate humanity from all its problems] - postmodernism challenges all these). Postmodernism challenges all these but it also incorporates them e.g. postmodernism doesn't get rid of the belief in the "new" (as in advances in technology etc) but it meshs it with the old. So you have a mixture of traditional values and idea and the new. You can see this sort of thing coming through in post-colonialism where one of the key tenets of most of the texts is to reconcile the differences between the traditional way of life and the new way of life introduced by the foreign powers.
Most of these sort of things are intertwined with each other these days.
