Co-loc2 is a pretty good plugin. It will give you Manders and Pearson's coefficients, which are the two most common reporters of colocalization. Here is a link to the Wiki https://imagej.net/plugins/coloc-2.
I also highly recommend the paper "A practical guide to evaluating colocalization in biological microscopy." Am J Physiol Cell Physiol 300: C723–C742, 2011.