What makes certain rainclouds gray?

Well, to answer your question, first let’s start with why the thunderhead appears white. This is due to Mie scattering. Mie scattering refers to scattering of light by particles (in this case, the water droplets that make up the cloud) that are much bigger than the wavelength of light hitting them. Cloud droplets are excellent at scattering all the wavelengths of visible light, so when we look at a cloud that is far away from us, we see all of these scattered wavelengths at once. This is why clouds appear white. The dark grey you see occurs when the cloud is above you, or in between you and the sun. As light enters the cloud, it is scattered in all directions, and for sufficiently thick clouds, not much makes it out of the other end (the end you are viewing). Just as white is all colors (or wavelengths of light) black is the absence of color (or lack of wavelengths of light). So what you are seeing is the cloud scattering most of the light, which makes it appear dark.

A ha! So rain clouds (as opposed to fog or the fluffy cumulus clouds), because they contain larger particles of water, are more dense, and thus scatter light more efficiently?

The more daker the rainer.

Most dense clouds look darker and visa versa.