>>>It's a bit strange for windows (assuming it is) to randomly change the background colour
Yes, that thought had occurred to me as well (but I'm never too surprised when Windows decides to randomly change something!).
To check your monitor's display, click here:
http://reeddesign.co.uk/testcard.htm
All of the numbers from 0 to 10 should be visible on both numbered lines.
The first coloured square should be a vibrant mid-blue shade. The 5th one along should be a vibrant (not pastel) light-blue.