Chandab, your colt's pics are very different, I've never seen such a color change. Cute boy!
Sarah is right, most bays are born with light colored legs, and frequently have white or light colored hair in their tails too.
This pic is of my bay gelding as a weanling
Here are two bay foals born this year; both are clipped (but the one on the right hasn't had her legs clipped). Look at the difference in color! The one on the right clipped out nearly black, which most of my bay foals do as babies.
This pic shows them too, you can see how differently they are shaded.
Now the other filly, she is actually a silver bay. Yes she is! But she is a silver bay roan (probably not true roan, just roaned out alot) sabino appaloosa.
Ok, duh, I forgot to put that last picture in!