I use Humbrol for my detail and brush painting and really like it.
All my Humbrol has been bought in the last 3-4 years and I've had no complaints. I haven't bought white though, so i'm not familiar with the yellowing. I have an old jar of Model Master white which, when left, settles out of suspension and the carrier base definitely has a yellow tinge. When you mix it up again though, it's nice and white.
I'm intrigued now as to how good Humbrol must have been back in the day.