I had a white blazer prior to getting my H3 and the Blazer never had the issue of yellow spots.
Even though they are both GM vehicles clearly they use different paint and/or clearcoat.

I'm having the dealer detail the exterior for me on Monday. To do a wash and clay bar is only $99 + tax.

 

I dont belive this is caused by rail dust. I have a 06 H3 that is white and i had the same problem , I took it to dealer and was told the same thing. They will offer a 1 time clay bar treatment witch i accepted and it reappeared 2 months later.
So i began to think, ................ What I found to be MOST affective is to replace the cheap semi metallic brake pads with quality ceramic ones. I belive the spots come from the brake dust scinse most of the problem is concentrated on the sides of the truck to me that was the most logical cause.
Well my solution worked, I haven't had anymore mysterious spots on my H3 at all after doing this.. Remember DONT use semi metallic pads use only Ceramic ones there a little pricey but well worth it..

Good luck