A while ago when I purchased a lot of Mimi and the Goo Goos I happened to notice a slight difference with one so I checked them all and found a number of variations, some quite easy to spot, others that hardly count as variants.
One of the differences that runs across all figures is that some have dark blue eyes and some have light blue eyes and then there are differences in colour tone that match with the eye colour - the figures with light blue eyes tend to have lighter, brighter colouring than those with dark eyes. As I've seen it a number of figures I think it is colour variation from the production line rather than colour alteration with age.
The figure on the right has a lot more pink in its mouth and the pink is a bit brighter. This isn't so much of a variation as a difference in age, paint colour runs and the enthusiasm of the painter.
Birthday Cake figure:
Here there are both eye colours and the figure with dark blue eyes has a much different colour purple. The pink and yellow are also more muted but that's a more subtle difference.
The biggest difference I found - in the figure on the right the teddy bear has a lot more detail (a face with eyes and nose and white 'stuffing' is visible), the hanky is coloured white and there is a green stripe running along the bottom of the shirt.
The figure on the left has much darker eyelashes/eyeshadow (whatever it is supposed to be) and has the more muted colouring when compared to the figure on the right.
Soapdish figure:The figure on the left with dark eyes has stronger colouring than the figure on the right which has light eyes.
Icecream figureThe figure on the right has dark eyes and the edging on its bib is much darker in colour than the figure on the right.
I've got to decide wether these differences are different enough to be true colour variants or not. If not then I am going to have more spare figures! I don't think the burger one is but I can't decide any of the others!