If you have a bad roast, even the most perfect Kona Bean can become sludge. If they warm up too fast it leaves some charred or burnt in appearance. This adds bitterness to the end product. Warm them up slowly to heat the colder middle of the beans. It takes longer but the rewards are worth the wait.
Some beans are stubborn and want to stay green in color.
Be patient and they will eventually turn to the right shade of color. You're warming the beans, not cooking them yet. The beans will go through a metamorphosis in the next 10 minutes. They turn brownish, then black veracious veins appear and the look done for a Medium Roast. They try to trick the roaster man and make him drop the load.