I built a bow a couple of years ago out of osage and the wood is not turning brown under the glass making the bow two tone.  I have two bows from the past that the osage under the fiberglass is the same color as the exposed wood.  I would prefer to have the bow look like it doesn't have glass backing.  Have they started putting a uv protector in the fiberglass now and if so, can you get it without.

I thought I would try to add a picture of the all osage and glass bow.[attachment=1]

Were the lams and the riser from the same tree?  Just inherent differences in pieces of osage would be my guess.
There is variation in how the color of woods change over time from individual to individual tree.  I've noticed this big time with cherry.

I dont know if glass has uv protection but I known not all osage is equal the 2 staves in the pic where both cut in Sep 2016 from 2 diferent trees and have different colors ,I must have 40 osage staves aging on the walls and in the rafters and they all patina at a different rate or different colors to start , I dont know if your lams came from the same tree as your riser ? [attachment=1,msg2826630]

No, the riser and the lams are not from the same tree, but the overlay under the leather wrap is left over from the lams. 

Some 'glass has UV protection, some doesn't.  You have to look at the spec sheet.

Thanks Ken