The Idea is that you get only the color wool that's the same as the color from the sheep you have captured.

Unfortunately the way that Woot learns loot drops, it cannot differentiate between the different sheep types. Basically I only capture a Minecraft Sheep - the color of the sheep isn't know to me.
Granted that is possibly the fault of my design choices :)

There are things that I can do to handle the sheep as a special case.
I'll keep it in mind for a future release and possibly try and patch this case in, since we are talking vanilla mobs here.

how about some precision modifier which can be programmed to a target mobdrop so that only this item will be generated. the chance to get the target item could increase with higher upgrade tiers, or each tier gives the ability to set an additional fixed mobdrop

this would have the benefit that one could set things like target woolcolor but also avoid lots of trash when you don't need the other mobdrops