This really is an excellent question. Although the two terms are seemingly interchangeable at times, I understand colonialism as involving the implementation of colonies, whereas imperialism is a broader phenomenon describing how an empire manipulates and exploits its subjects.

For example, here is a quote from Stephen G. Gross describing some imperialism:

‘German imperialism evokes images of military aggression and ethnic cleansing. Yet even under the Third Reich, German imperialism has also worked through more subtle processes of economic and cultural penetration. […] This informal German empire was much more than just German; it involved German minorities as well as non-German elites from across Southeastern and Central Europe. And it gave Germany access to the markets and resources of half a continent. This empire emerged, however, not through guns fired. Instead, it arose through the work of businessmen of all kinds who manufactured products demanded outside Germany, through private institutions that engineered development programs, and through professors and students who became messengers of German ideas. It combined the export of goods and culture. And it was built with a kind of influence that can be called soft power.’

Traditional colonialism, in contrast, was a specific manifestation of imperialism that made space for settlers while superexploiting natives. See Transitional Economic Systems, pages 22–33, for a good example of this. The descriptions of the Fascist colonization of Poland are quite graphic.

I hope that this helps.