The game doesn't seem to generate any of these truely randomly in fact. There's something predefined that makes the draw happen the same way from one game to another (at least it does in my case). As it is right now, some designs and colours are much more common than what they should be. If you think sinople (green) is too rare, all I can tell you is that it's already three or four times more common than what it really was (furs were more common, and they aren't even ingame). Mainly because in medieval symbolism, green is commonly associated with madness. Of course gold (yellow) as colour of sovereignty, or gules (red) as colour of wealth are much more common, it really is not that surprising at all on a global scale.
Anyway I've been working on giving most dynasties their historical arms so the results should change a lot. Although if variety is really what you seek absolutely more than realism, I'm afraid it's bad news (heraldic groups are much more repetitive on a regional basis).