Whether a cloud appears white, grey or black has to do with light absorption and reflection. The greater the absorption, the darker the cloud. The less absorption and the greater the reflection, the lighter or white it will appear. Absorption is related to thickness of the absorbing media, the cloud and the nature of the moisture droplets therein. Less spacing with finer droplets as in fog, the more light absorbed. If heavy rain is present it can appear quite dark with minimal reflection and lots of absorption. If you are looking at a cumulonimbus cloud in the distance with the sun behind you it may appear quite white as in the example below:
If you are beneath it looking up through all that dense moisture it can look pretty dark. In the case of a gust front, the darkness can be caused by dust swept up and carried along in all that violent turbulent wind before the cloud.
Regarding wind, frankly, you've got me. Cumulonimbus and shelf clouds have the potential
to toss off violent, rapidly changing wind in terms of velocity and direction, hail, tornados, bursts, even extreme uplift at altitude. They also can very commonly kill the wind entirely at some point. Cloud suck really sucks for hang gliders and paragliders. Doesn't seem to be an issue for near sea level kiteboarders fortunately. We need to assume cumulousnimbus and shelf clouds have the potential to kick our butts and react defensively and early on too to land and secure. It is less clear when strong winds will be present and when they won't simply by looking at the clouds. Interpreting doppler radar, soundings, realtime wind upweather and other considerations seem to be necessary to attempt to accurately forecast whether a given cloud is going to whump you or not. In practice cumulonimbus clouds bring more violent winds very commonly in my area with shelf clouds bringing the same thing less often. This doesn't mean the odd shelf cloud won't slam you with 60 mph or even 120 mph winds from a burst because they will. Behavior may be quite different in other areas. Best advice, ALWAYS assume these clouds can mess you up, even kill you and react properly in advance. 747's won't fly near them but we will using a massive drouge-like device the size of a car without an airframe, engines, safety systems or much else. Sounds stupid in the extreme to me, not much theory in this given all the accidents over the years.