Gary:
About your vacs:
Those are Electrolux Diamond Jubilees, and they're notorious for yellowing. The unyellowed one is the oddball, not the one that yellowed. That beige was very prone to discolouration, for some reason. I have an Elux 1205 that is ten years older than my Diamond Jubilee, and people who don't know collectible vacs think the 1205 is much newer than the DJ. My DJ didn't even have the grace to discolour evenly; some of it's beige and some of it's tan.
It may be a problem with light, but I don't have fluorescents or halogen in my house (hate the bluish/greyish/whitish cast of the light, even the "warm" versions) and I have the same problem you do on my Diamond Jubilee.