You may notice in some RPs when he is older, i mention that his fur is dark and/or navy, save for the tip of his tail that remains a bright blue.

The reason why is because of his environment and emotions. If he is growing up in a negative environment (in an extreme war, strained home life, sickness etc.) his fur pigment darkens. Mainly because he's looking for a place to hide and the fur will fit his personality. Keep in mind that this isn't instant, he has to grow up in these conditions constantly for at least fifteen years.

If he stays in a mostly positive environment (a good group of friends, his brothers getting along, exciting adventures, happy atmosphere etc.) his fur will remain bright. He doesn't have to hide, and he wants to be able to be spotted from a crowd. It fits his optimistic personality. This only comes from a positive atmosphere constantly for fifteen years at least.

No matter what, the tip of his tail will remain a bright blue. Either sticking out like a sore thumb if he grows up with dark fur, or blending in with the rest in a positive home.

The longest it takes it eighteen years. After that time his fur will either be permanently that color or it will take forever to reverse. Like if he stays happy after getting permanently dark, the best you can get is about Sonic's blue. If he stays sad after getting bright fur, it will remain a hue brighter than Sonic's.

If there are stressful patches in a happy growth, he may end up getting patchy calico fur, mostly bright with dark splotches. Vice versa if he earns a sad growth. Again, the tail tip will refuse to change.