OP just called white people “normal people”.

My theory: Human Big Macintosh and Pony Big Macintosh were switched at birth.

 
Because it would look awful.

Allow me to refer you to Disney’s Peter Pan. Ahem WHAT MAKES THE RED MAN REEEEEEED.

He dyes his fur.

The same reason Thunderlane doesn’t show up  
black pony -> comically discolored black person  
red pony -> comically discolored red person

Just like Luna, Silver Spoon, and Cheerilee, Big Mac’s coat color is too deeply saturated to be a reasonable-looking skin tone, so they had to lighten it a lot more than the other characters to make it look good. For Big Mac, that meant making him very light pink, or ordinary flesh colored.

Human-specific albinism?

if he had his Skin the same Colour as his Pony Self Fur he would look like he will have a Bad Sunburn

The human skin tones are actually a lighter tone then there pony counterpart’s fur.

@Itsthinking  
@anonymouspegasus  
@PonyPon  
To continue with my previous post: hair and fur have the exact same chemical makeup and structure so from a chemical standpoint they’re the same. What most people tend to forget about horse “hair” is that if you were to look at a horse, the “hair” sheds and doesn’t grow past a certain length. This is partly evidenced by the fact that if you were to brush your hair clumps don’t come out of it but they will if you brush a horse.
 
As for how I know this: a little internet research plus 15 or more years of my aunt owning a horse/pony.

@DashingJack  
According to a mammalogist of the American Natural History Museum in New York City, as interviewed by Scientific American, there really is no difference between hair and fur. For the most part, people associate fur with it being softer and shedding, while hair is coarser and doesn’t shed. However, horses shed yet the hair/fur is coarser, so in this case they could be used interchangeably.

Principal Luna isn’t dark blue, either. And I think they lightened Cheerilee’s body color as well. Though I guess that means Big Mac’s body color should have been pink…

@PonyPon  
*then

@DashingJack  
Hair keeps growing, fur grows to a point and them sheds.

@Itsthinking  
So, what is the difference between hair, and fur?

@DashingJack  
Yeah, they based their skin color off of their coat color.
 
I’m just saying that horses don’t have fur. Their coats are covered in hair, not fur.

Pony Big Macintosh -> Closest to normal Horse
 
EqG Big Macintosh -> Closest to normal Human

A deep red skin like that would make him look like Satan.

@Itsthinking  

Oh, and ponies have hair, not fur.

They’re not exactly the same and dark colored characters would look terrible.