Aberdeen is one of the sunniest places in the UK (note, I didn't say warmest. :) ) The Grampian bus is in the old colours from when the buses were run by the council , as a 'throwback', not privately run by First Bus (which is now a worldwide company, but was started by ex Grampian bus drivers in Aberdeen, and still has its HQ here). You saw only buses because the section of Union Street you were on prohibits cars. And lastly, Union Street used to be the main shopping street, and used to be very bustling and crowded, but the 'city centre' is now split up a bit, which combined with the global issue of dying city centres, makes it feel extra empty now.
Other than how FirstBus came into being, I knew the rest. I had a sit down with a taxi driver a few months ago and he explained exactly how and why for Union Street. I've seen the old pictures of a jammed Union Street and it is such a massive difference. Thanks for adding some clarity. It is greatly appreciated.