R_gr vs. the number of pixels contained in candidate green sources. To determine the values used as selection criteria in our detection algorithm, we apply different combinations of these values to a known sample of green source/maser pairs to see how many the algorithm recovers. The grayscale shows the percent of known pairs recovered for each set of values. In our algorithm, we use a minimum number of 9 pixels and R_gr   ≥ 0.2 as the cutoffs for green sources (marked with a black + in the image). With these values, we recover 13 of 14 (93%) green source/maser pairs.