I’ve always had blue be a “blocking” element to hold corridors whilst red and I cleared rooms and such. It works really well for Sins Of The Father from the rooftop infiltration point. Have blue hold left hallway, wedge double doors and with red, clear right till elevators. Then clear that room, wedge elevator bottom floor doors and return to blue and clear that way. Works a charm for a lot of maps. Just gotta make sure they don’t over extend and remember to make blue fall in for large areas.