I have no idea about the color coding
but generally the idea is to dump blackwater first, then chase with grey water to rinse the hose
blackwater tank would likely have a larger pipe coming to the drain.
and the blackwater tank would be under the toilet.
make sure the black tank is fairly full before draining
then connect the drain hose to the dump
then open the blackwater
then when that's done close it and open the grey water
The rinse would be a nozzle inside the tank for rinsing it out after. No need really in doing that every time (or at all really). Plenty of us have RV'd for years without one even installed.
I suppose it could also be a way to fill the black tank that's not very full before dumping it if you have to
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