@Aru  
Octavia and some of Pinkie’s relatives are grey

ponies should generally have varying colors, but not colors that are excessively vibrant or clashing most likely.

@Background Pony #577A  
A lot of ponies are also like the OC on the right, which is also subpar, although not an abomination like the one in the middle. Consider:
 
The coat is bold. The cutie mark is a number, and matches nothing else in his design. The teeth are sharp and edgy. The hair is black–probably because the creator has dark hair and doesn’t want to represent himself as a colorful pastel horse with some weird mane color, and thinks that this makes the character look more “him”.
 
The mane of the pony on the left is a bit too dark for my liking, too.  
in summary, though this panel may give decent basic advice, I fear it would not help those who already know not to make alicorn OCs with millions of accessories.

@Aru
 
It would probably be fine as long as there aren’t fifty shades of it.
 
Because that’s too many.

@Background Pony #577A  
What would you say about a pony that was nothing but grey? (different shades of it, at least)

PASTEL COLORS ARE YOUR FRIENDS

@Background Pony #577A  
I think the OC creators are just trying to be, well, “original”, by having switching the coat and mane shades.
 
Doesn’t excuse the GRIMDARKEDGY OC ponies that look like they were dipped in an inkwell, though.

Something I’ve noticed: most (but not all) ponies who appear in the show have a darker mane color than their coat. A lot of OCs have darker coats and lighter manes, or their entire color scheme is dark.
 
It’s something subtle that I think goes a long way. And it’s a bit more useful than making fun of all the red and black alicorns.

No red/black = fail.

I find the little yellow pony very cute