What causes this coloration?

[aznboi3644]

Here is a sycamore stave that has been drying for 5-6 months with the bark on.  The back of a freshly cut and debarked stave is pristine and white.  After drying with the bark on the back has this beautiful coloration of white spots and a darkened grey look with the little pores in the grain popping real nice. 

[PatM]

 Fungus.

[Hamish]

Could be fungus, if those spots are softer and punkier than the surrounding wood. If so then it probably won't be good for a bow.

Sycamores, and some other white woods, have a reputation for discoloration when cut in the summer. This is due to the high content of sap, and perhaps some sort of fungal bluestain. It supposed to be only cosmetic.
You can usually get around it by cutting in winter when the saps down.

I have tried wiping the surface down with alcohol, and with bleach on whitewood cut in summer, during high humidity.  Then sealed with PVA or shellac, and the grey/ bluestain still happened. It happens very quickly, in the first couple of days.

If the spots aren't punky, they could be due to the chemical/fungal reaction not occurring in localised spots. If the wood was cut with low sap and low humidity, the white would  be how you typically expect sycamore to look like.

[DC]

I'm surprised that the bark came off that cleanly after drying for 5 months. Usually Maples bark is like it's glued on. I would be suspicious.

[aznboi3644]

DC this bark was not easy to scrape off at all. Lol I worked my butt off scraping it away.  I’ll know next time to not wait.  It was so dry and hard.

The discolored wood is just as dry and hard as the sides and belly. I feel zero difference in the wood.  I’m thinking it has to do with the sap or something. 

[bownarra]

I've seen spots like that on Sycamore when the stave was cut and then bashed about a bit getting out of the woods. If cut when the sap is up and the bark slipping very easily.

[BowEd]

I'd cut a cross section through one of the discolored places and see what's going on from the end grain view.More than likely no difference though since you said you don't see or feel any difference already.