I've had the same experienced with albums where some of the prints are perfect, but others have changed to a brown/reddish colour. There were a lot of labs competing for business in those days and probably some cut corners and used stale chemicals or didn't wash long enough or fix properly. But equally suspect were the papers - there were more manufacturers around then, and some of the papers possibly had very unstable dyes etc that have failed to stand up to the test of time. Unfortunately colour negatives can change over time as well. It might be possible to scan the prints which have changed and use a photo editing program to make the colours more realistic. I have had some success with that, on the few that I have so far worked on.