I wouldn't want black contents in my gray tanks either. The lines for gray are usually smaller, 1-1/2" or 2" versus 3", and not designed to handle the solids and paper. But in practice, that isn't likely to happen unless you do so on purpose. Depends on how you use the tanks, and how they are plumbed in a particular unit, but if you give it some thought it won't be an issue. Just keep in mind that contents are not going to be sloshing back and forth between tanks. They are going to find a common level, the flow is slow, and that's about it really.
