True, but I've nothing against dark-coat/light-maned OCs (as seen here) as long as you only have a few. Of all the canon ponies, only two have coats darker than their mane (Big Mac, P. Luna), so it's not impossible, just uncommon.
What I mean is that not very many people seem to pick colors for their OC's coat/mane/tail/eyes/CM that mesh well together, or they pick too many (or too few) colors. Just in a color sense--not mentioning all the self-indulgent general design bullshit that a number of people seem to revel in (although to be fair, that's a smaller amount of people than the amount of people with a poor color sense).
It is. It's not something you'll see a lot of, I'm sure, but it's totally possible that you'll see it sooner or later.
Not...really. What colors you like is subjective. What colors work together...not so much. It's still subjective but it's not 100% subjective. When it comes to pony OCs, the colors you choose should work together visually well (or not, if you're trying to draw attention to something, whether because it's weird, it adds flavor, or it hints at something unusual, i.e. they're half-gryphon or something). Like in your case, Black Bolt has red eyes because he's 'corrupted', I believe? The red eyes pointedly don't mesh with the rest of his scheme because of that corruption, I would imagine. As much as I would like to crusade against black and red being the 'evil' colors, using red eyes to indicate corruption/evil has been well-established and works.
The other thing when it comes to colors is your OC's personality. Flashy or outgoing ponies will have colors outside of their eyes that grab your attention (Rainbow Dash, Pinkie Pie). More core-focused, internal characters have their attention-grabbing colors balanced between eyes/coat/mane (AJ, Twilight), while subtle or introverted ponies only have one odd color that doesn't immediately mesh with their color scheme--their eye color (Rarity, Flutters).
The way I did my OC was to contrast a bit the change between his corrupted version and normal version. When I created Blue Thunder I toyed around with blues for coat until I found what I would call the standar blue, not dark nor light. For the mane I chose light yellow, more or less like Derpy's mane color. The eyes I was debating between brown or green, and I sided with green in the end because it seemed to suit better.
Then when I came later with the idea of a corrupted self, I decided to just darken the coat and light the mane. The eyes were a special case for that one.