It’s pretty common when hanging crown molding that the joints don’t always come together perfectly. Sometimes a slight visible light-colored line results where the unfinished molding shows through at the joint. In order to make this less obvious to the casual eye, I have found that marking both inside edges of the crown where they will meet, using a black marker or a similar wood-colored pen, minimizes the mismatched seam. Once the joint is assembled, most people will never see the gap.
– Dale Miller