Reflectivity vs transparency is essentially defined by what happens when photons from a light source interact with the material.
If the photons pass through the material without being absorbed by any molecules, the material is transparent. If the photons bounce off, it’s reflective. The more unevenly the photons bounce off (e.g. because the surface is not perfectly smooth) the more the light is scattered in all directions and the material appears white.
Colours come about because for many materials, photons with different amounts of energy–i.e. having different wavelength, which we perceive as colour–are absorbed or reflected differently by the material:
For example, a red material might absorb any high energy photons but reflect lower energy red photons. When those reach our eye, we see red. A green material would absorb the red and blue ends of the spectrum but reflect the green in the middle. (There are other ways that colour come about but that’s the gist of it).
Something that appears “black” to us does so because all wavelengths in the visible spectrum are absorbed rather than reflected.
