I have mixed feelings on what causes osage to turn. Sunlight definitely speeds up the process but raw wood exposed to air and never in the light will turn just as dark or darker than finished wood left in the sun. I have staves under my house in a crawl space that is pitch black all the time that are dark purple on the outside.
Here is a bow that has never seen the light of day for more than a hour or so in the last 10 years. I made it for a brother-in-law who loved such things but had MS and only shot it once before MS robbed him of the use of his arms and it went on the wall. It is much darker than the picture shows.
