http://www.mspaintadventures.com/?s=6&p=004199
Please look at this page. Do you see Karkat’s blood? It’s not bright red, now is it? It’s a bunch of multicolored, flashing question marks. 
It is revealed to be bright red in the next couple of pages.
Now think of this. All the other trolls’ blood colors are openly displayed, when first revealed, Karkat’s blood color is ambiguous. 
This shows that not everything in the images are as they seem.
It’s completely possible that our human protagonists’ skin colors are like Karkat’s blood, with the exception of them never being revealed. There are certain human’s that have defined skin colors. They are special exceptions. 
With all that said, you should just stop worrying about it. It doesn’t cause any harm to anyone. In fact, yellow, glittery Calliope looks pretty swell.
~Card

http://www.mspaintadventures.com/?s=6&p=004199

Please look at this page. Do you see Karkat’s blood? It’s not bright red, now is it? It’s a bunch of multicolored, flashing question marks. 

It is revealed to be bright red in the next couple of pages.

Now think of this. All the other trolls’ blood colors are openly displayed, when first revealed, Karkat’s blood color is ambiguous

This shows that not everything in the images are as they seem.

It’s completely possible that our human protagonists’ skin colors are like Karkat’s blood, with the exception of them never being revealed. There are certain human’s that have defined skin colors. They are special exceptions. 

With all that said, you should just stop worrying about it. It doesn’t cause any harm to anyone. In fact, yellow, glittery Calliope looks pretty swell.

~Card