I have had mixed results with the dark, oily stuff. Some has made the best bows I ever made, some just wouldn't respond well to heat straightening and would crack easily when I tried to make a correction, made real sluggish bows as well.
Over all I would much rather work with the butter colored, hard as a rock stuff. It heat corrects easily, holds the correction well and is the most durable of any other osage I have. Always a consistent, low set, high performer as well.
Then there is the extra light colored , chalky, low density stuff, not very good for selfbows but will work in a bamboo backed bow.