On the ones in the photo, I first soaked/washed the hollowed out bones in Dawn dishwashing soap to "degrease" them - they won't take stain if they are greasy. I just soaked the bones in a glass dish with potassium permanganate (the active ingredient in Old Bones) - didn't measure, just sprinkled some in and put in water. The mix turns purple and I soaked the bones about 2-3 minutes, then dried them and glued them up!
Your Humble Servant,
Just Dave