My mosin has yellow spots on the bolt, what is it?

I don't know. Have you tried wiping it off?

You don`t have a picture so it is difficult to diagnose those yellow spots. I will tell you about my experience with a similar Mosin Nagant problem: I bought a M-38 about a year ago and found some of those spots along with small residue that looked like small rectangles and when I cleaned them off they left spots. I suspected that corrosive ammo was used and then on top of that the firing pin was set way too high and possibly causing a primer rupture. After hours of cleaning and complete disassembly of the bolt group I reset the firing pin height with the M-N multi tool and fired some good Prvi Partizan ammo through it and never saw the likes again.

Simichrome polish cleaned the bolt parts and now I have a rifle that is a good shooter. Also,a little polish through the bore brought out a luster that I began to lose hope that I would never see in that particular rifle.

It sounds like a very early formation of rust. If you weld and expose the weld to watter you will find the weld will match your description. Take it to a gun smith before attempting to do anything about it. He may provide you with an excellent method to remove it.

It is industrial grease. If you want wear a latex glove and remove some. Once you do roll it between your fingers and you will be able to tell that it is thick and sticky. Also the grease should have a strong smell. My friends Mosin had grease all over the bolt.

i'm guessing it's some kind of oxidation. you coated the bolt with something recently, and it's reacting to air, moisture (humidity) and human skin oil to turn yellow. you cna try polishing/scrubbing it off with some cleaner and 0000 fine steel wool.

its agent orange quickly throw it in the nearest dumpster