You are probably grossly overexposing the plates. Here is a test strip sequence I shot recently using a passport camera. The exposures are all at the same aperture (f/11 IIRC) for 2, 4, 8, & 16 seconds starting in the bottom left quadrant and going clockwise.

Because there really is no true black anywhere on your plate including the islands the collodion pour didn't cover, I'd say you have too much developer activity as well. (Compare to the black corners in the plate above.) Usually that's from trying to "push" the development but since you state development was 15 seconds, then the developer is probably too strong. You actually say "approximately 15 seconds" and if you are getting out to 20 seconds, you are overdeveloping. Develop exactly 15 seconds every time and tune your exposure and chemistry to that exact time. (You could use a different time, say 30 seconds, but then you'd have to adjust your exposure and chemistry to that time. The point is to be consistent.) Try diluting a small batch of developer with 50% more distilled water and see if that gets rid of the fog on the unexposed parts of the plates. Sometimes chemical fog results from developing in a tray that is dirty and has silver deposited on it.

If your silver bath is made correctly 3.5 minutes sensitizing is probably OK. Some of this may be from the plate soaking too long in the silver bath. Check the progress by noting when the fluid runs off the plate in an even sheet rather than in rivulets. At that point, the plate has sufficiently sensitized.

The color shift may be a result of an insufficient wash. As with the silver nitrate, wash the plate until there are no visible rivulets draining the plate. Then, wash it further in clean water for about that same time.

Sometimes the plate starts to dry out and the drier areas will not have the same color as the rest of the plate because the chemical action is different. But, I don't think that is the problem here. I think it is the brief wash time. Another possibility is exhausted fix which tends to give yellower plates. Which fixer are you using?