Would giving a talk in a colloquium aid my application? I'll probably give a review talk about current discoveries in applications of mathematics (not my autorship) on the colloquium series of my school soon (small school, accepts undergrads for giving talks). Would this help in any way my application for PhD programs in applied mathematics?
That depends on the graduate school, and the competition. So it is hard to say. However, giving talks is part of the day to day life in academia. So getting practice early won't hurt once you are in the graduate school.