Signature ranges: Signature range tuning is probably the most effective method to improve detection accuracy. Each color signature (1 through 7) has a slider to adjust the inclusiveness of the corresponding signature. Slide the slider to the left if you want the signature to be less inclusive (you're seeing false positives) or slide it to the right if you want the signature to be more inclusive (you're getting false negatives.) There is more information here.

Min brightness: sets a minimum brightness setting for all signatures. That is, if a pixel is below the minimum brightness setting, it won't be considered as part of any color signature. So if you are seeing false positives that are sufficiently dark, you can adjust the Min brightness setting higher to reduce the false positives. If you are not able to detect objects that are sufficiently dark, you can adjust the Min brightness setting lower to reduce the false negatives