Usually, a word that has a segol such as "zecher" gets a tzerei when it becomes an adjective. Example - "mateh" becomes "matei" when used as an adjective as in "matei Levi".
Q - in the phrase זכר למעשה בראשית is the word זכר considered an adjective? On the one hand, I would say, yes by the fact that it has a tzerei. However, I am not sure because the next word starts with ל (meaning "to" or "for"), which is a preposition.