Is there a term for a colony effect in language change?

This might be a better question for the Linguistics Stack Exchange site, but I'll give it a shot. Language divergence or linguistic divergence is "when a language breaks into dialects due to a lack of spatial interaction among speakers of a language, and continued isolation causes new languages to be formed."

The branch of linguistics that deals with this is called dialectology:

Dialectology treats such topics as divergence of two local dialects from a common ancestor and synchronic variation.

Dialectologists are concerned with grammatical features that correspond to regional areas. Thus, they are usually dealing with populations living in specific locales for generations without moving, but also with immigrant groups bringing their languages to new settlements.

There is also the term dialect continuum -- "a range of dialects spoken across some geographical area that differ only slightly between neighboring areas, but as one travels in any direction, these differences accumulate such that speakers from opposite ends of the continuum are no longer mutually intelligible."

The linguists use the term

conservative

for the phenomenon of where a language preserves older features, rather than creating new ones (as is often the case).

But it is not a universal that the colonies are more conservative then the home culture. Italian is the most conservative in Romance, and it is the geographically divergent members ones that have the most changes (French and Romanian).

As to English, RP is non-rhotic which is supposedly a newer innovtion. But in America... mostly rhotic but some Southern accents are non-rhotic, mostly corresponding historically to West country accents. So out of all that, how can you really say which one is being conservative and which not?

I think a term such as "fossilisation" is what you are looking for. But I'm not sure that it is real in the context you are talking about, though there is certainly a perception that it occurs.