Jack's pic above and on my avatar - it's lighting. I have a jersey from the 73-74 years in the correct royal blue with tackle twill yellow numbers.
Some other team jerseys get that lighting variance in old pics. The old Charger photos have that illusion more than any other team. They started off royal, but some pics it looks darker, then they went to a lighter blue (some call it powder, I call it sky) in the late 60s, but in some pics it looks washed out, then to darker royal in the 70s.