Hat tip to Bisrepita for the earlier share.
In my experience, red tops can do almost anything they want -- they can deviate from the planned route in any way that they wish. Also, most only accept cash (is this changing?). Green tops are pretty strict about stops and accept cash or local metro card (Octopus). On a deeper, urban explorer level: The red tops have waaaaay more aggressive drivers. It feels like GTA sometimes.
When riding a mini-bus, you only need two words of local language (Cantonese) to make it stop: 有落 jau5 lok6 ("yau-lok"). (You need to really shout to be heard over the revving engine.) For green top routes, use Google maps. They will guide you on what green top to take. Example: If you want to go hiking in Sai Kung, take the 101M green top mini-bus from Hang Hau metro station to Sai Kung pier. (Google maps can provide directions with the bus info.) Red tops are more adventurous and should only be taken if you speak/read more than a few words of Canto (50-100 words is fine).