Can anyone shed any light on this phenomenon? There is discolouration wherever a cream colour appears on this particular sample tin. Is it oxidisation?

This is one of mine and it’s starting to discolour.

This is on Ebay. Does anyone have one without the discouloring?

Doesn't look like like any thing wrong with the colour .. That is the original colour of the tin .. Only the chipped sections are showing  signs of rust . Your tin is a quite reasonable example of that tin . Looks like it could do with a good polish though . Get some advice before you go at it .

I'm with Relics, I don't quite see what you mean. Your's looks typical. I have a similar one.

Here's a couple of good ones from my archive, not mine.

Thanks guys. It just looks odd that the tops of the tins/packets in the pictorial on the Ebay image and yours are black, and on mine they’re not, but looks like it’s starting to go black. Maybe I didn’t explain myself very well.

The tops of the packets are silver in colour .. to depict foil or earlier, lead in the packaging .
The silver on the tin is often the first to fade / degrade . Not uncommon . Just another factor to consider when buying  .