I am not sure how to formulate definitions in terms of tenets. Colonialism and Postcolonialism refer to certain periods in history relative to the countries being discussed.
Colonialism is often discussed as the history of imperial expansion and colonization which was generally initiated during the age of exploration. Colonialism is usually discussed as a European domination of American, African and Asian lands. This is true, but this could also include examples like the Roman and Persian empires. Empires that have taken over other countries have often established colonies as a way of staking out territory. In literary analysis, colonialism refers to literature and criticism dealing with the periods of colonialization. This can be from the perspective of the colonized or the colonizers.
Postcolonialism refers to the time after these colonized nations have become independent. Since these nations still experience the effects of colonialism, a postcolonial interpretation can often include the events of colonialism. A postcolonial analysis of historical or literary documents usually focuses on the challenges faced by the once-colonized and now independent people. The challenges they face could be issues with identity, race, economics, politics and culture.
If you’re doing a historical study of India or A Passage to India, a colonial study would focus on India during the British occupation. A postcolonial study would focus on India after its independence from Britain. Postcolonialism is more encompassing because such an analysis must address the events of colonialism to discuss its postcolonial effects. Culturally speaking, postcolonialism often refers to the interactions between Europe and the nations it has colonized. Particular attention is paid to the ways race and cultures are represented by the colonized and the colonizers.
If you generalize the concept of colonialism to mean “certain degrees of control by one country over another,” there are certainly examples today of colonialism in the form of imperialism.