I think they are both genuine tamagotchis. I don't think it is strange that there are differences in the colour of the paint and the way the toy has been painted.
These tamagotchis were produced in their thousands - when the toy was a craze all over Europe (and America and Asia).
They were produced by more than one factory, in more than one location.
They come from two different countries of manufacture (one is fromThailand, one is from Indonesia by the look of the photo).
I believe you bought them from different stores at different times - probably years apart.
There would certainly have been more than one supplier of tamagotchis even just for the European markets when they were at the height of their popularity.
It is not surprising that the colour doesn't exactly match - they would have used different product dyes in the different countries and it is possible that different factories worked to different minimum standards of decoration and finish - and probably also had different "quality control" checks also.
This does not make one of them a fake.
If you want to be more confident about them take them out of the packs and turn them over and compare the back of the tamagotchi shell (not just the box).
Do the words same words appear? (my older tamagotchis have the words "BANDAI 1997 MADE IN CHINA" stamped or engraved on the back of the shell).
Are you worried one is a fake because you want to sell it online?