In this post I will be tackling why in the world has sonic changed his eyes and fur..... that's pretty much it. Anyway, all things aside, lets make sega logic...... logical.

mobians have different ways of growing, some mobians have their eye color shown at a very young age, the ones who are lucky at least, I am referring to Cream the bunny, who already has her eye color. As for Sonic he got his eye color in at the age of.. I think 13-15 (in Sonic Adventure 2). Mobians get their eye color at a ratio of 6-15.

                                                                                                          Now on with the fur, some mobius like Tails start of with a dark color, while mobians like sonic start out with light color, then changes the lighting (depending on the lighting they begin with) as they grow, some mobians may not change fur at all.

Just wanted to get that out, even if the logic is small I will write something about it. Anyway, see y'all vizzed members later.  

Alternatively, you could just use the knowledge that it's just the higher amount of detail in the games. There were only a few pixels to allocate to Sonic's eyes, therefore the colour didn't matter. The colour likely comes from trying to make sonic fit in to a realistic looking world; people always make things look darker when it comes to trying to make things more realistic.

I wasn't putting this videos game wise, I mean of course they were trying to make sonic look realistic, I just putting a reason why he changed based on how a mobian grows.