Look into pure/ethyl alcohol extraction or pheonix tears/rso. Your first problem is 250 degrees is over the burning temp of thc/cannabinoids. 180-220MAX for concentrates. You can also look into rosin pressing. All of these methods should get you closer to where youre looking to be.
de-carb is mostly useless, especially if you're not focused on an extremely high-productivity setup, so don't sweat it too much. Do ethanol extraction, and boil (you'd better have good air circulation) with activated carbon to remove large organics (literally until the dark brown/green coloration is gone) that will leave easily burn and leave nasty residue, then move on to evaporating or whatever your chosen technique before re-suspending in your medium of choice. That will probably get you the cleanest end product without too much work.
Yes, the color, but, more importantly, the stuff giving it the dark coloration. It's long chain organic molecules that will probably decompose at the same heat that it takes to vaporize your terpenoids. And no, cannabinoids won't get caught in the activated carbon at a rate that matters. End all those sentences with "probably." Still, purer is always better, right?