New Front Tyres
Since writing this post, I've replaced the front tyres with another set of Mastercraft white walls. I found that the new tyres started to get a similar yellow stain on them as soon as the blue protective coating was washed off. Interestingly, the rear tyres (which are 10 years old) have perfectly white stripes, there's no discolouration at all.
Being so old, the rear tyres were made in the USA, whereas the old front tyres and the new ones I purchased are made in Mexico. My theory is that the quality of the white stripe on the Mexican tyres just isn't the same as the old US-made tyres. Has anyone else had a similar experience?