How can immigration and emigration alter allele frequencies in a population?

Let us say you have three alleles in a population of beetles. Two colors; brown is recessive to green. Thus you have; GG, which is homozygous dominant and green, you have Gb, which is heterozygous and also green. Then you have bb, which is homozygous recessive. This is your population of beetles.
What do you think the allele frequency would be if GG, the homozygous dominant, either immigrated, or emigrated out of or into your population of beetles? Since the frequency of Gb and bb would necessarily go down statistically you would see more green morphologies and a change in genetic allele frequency. Assuming normal conditions.