What features allow one to determine whether a mass is solid on ultrasound? that the area on sonography is grey vs black? or something else?

There are several: features that determine solid vs. fluid(e.g. cyst). As you point out, one is that fluid tends to be black, and solid masses tend to be grey or brighter. But not always. With doppler, a solid mass may have internal blood flow. Fluid will not. Fluid tends to cause posterior "enhancement"(brightness), and solid either nothing or "shadowing"(a dark band) posteriorly. It is a combination of features.