I do not know specifically about what's implied with French colonialism. For "effet boomerang", think simply as a boomerang, something that comes back to you. I would surmise that whatever impact French colonialism has had on such and such country, the effect came back to haunt France.
Yes scribod, "effet boomerang" means that something you did before comes back to you in some way, the nuance is, it's not usualy in a good way! And your guess about colonialism is perfectly right, it comes back to haunt us!
Hope it helps