@Unicorn colours
Heh, that reminds me of the fact that Celestia isn't pure white, either. Technically, she's a very light shade of pink.
As you can see, the outline especially is a dead giveaway. So that notorious Pinklestia toy was kinda half-right, in a way.
If you look at the other "white" unicorns, you'll find that most of them have a bit of a hue to them, as well. Rarity is actually light gray, Fleur has a touch of lavender in her coat. Fancy Pants seems to be pure white, but that's offset with the outline colour. Shining Armor has a pure white coat so the whites of his eyes are actually a very light shade of blueish gray to compensate. Same deal with Blueblood. Kinda hard to say anything definitive in regards to Vinyl, though, since she's usually seen through a colour filter of some kind.
I've always found pure white to be a bit of an unwieldy colour, personally. It tends to glaringly stick out and it looks inorganic and artificial. I like off-white much better, it's much more natural and blends in quite well. In fact, we Finns call it luonnonvalkoinen which, when translated word-for-word, means "nature's white".
As for why there's so many white unicorns around, well, that would be because that's kinda the go-to colour for unicorns. Has been since Medieval days in fact, at least here in the west. Before that, the Greeks basically used the word to describe the rhinoceros. (Fun fact: the word "unicorn" is a portmanteau of the Latin words unus, meaning one, and cornu, meaning horn. So, quite literally, "one-horn")
So how'd the unicorn go from rhino to horned horse? Well, that's not entirely clear, but as far as I've understood it, old pagan myths from the ancient Germanic tribes described a horse- or goat-like beast with one horn, a creature representing lust. It is no coincidence the story goes that only a virgin can tame it, you know.
Later on, when Christianity became the dominant faith in these parts, they had a habit of co-opting old pagan myths and traditions and rewriting them to better suit their ideals. Holidays like Christmas and Halloween are a product of this, by the way. Anyhow, the legend of the unicorn was one of these rewritten tales, being repurposed as a sort of allegory for the death and rebirth of Christ. Rather amusingly ironic, considering the original material.
The end result of this is a creature that paradoxically represents both the serene heavens and the untameable wilderness. A sort of allegory for the human condition, if you will, of higher mind in conflict with base instincts. Perhaps that's why people continued to be fascinated with unicorns long after dragons and griffins were relegated to children's stories and old wives' tales. Or because the Danes kept selling narwhal tusks as unicorn horns for centuries, take your pick.
Well, that certainly went off on a tangent, eh? Let's just say that unicorns are something of a passion of mine. Have been since I was a wee lad. Hell, I even keep a blessing of unicorns in any army I lead, if I simply can have them. Their healing abilities combined with their combat prowess really helps on long campaigns, you know.
Since we're on the subject and I'm on a roll here, why don't I make a recommendation for those of you who find yourselves with little to do in between seasons? Or hell, anyone who wants to see some classic, quality animation? Well, there's this here little gem called The Last Unicorn.
The more artistically inclined among you have probably already heard of this one, and maybe a few of you have had the fortune to see it sometime in your childhoods. Still, even if you do know it well, I'd recommend seeing it again. Can't have too much of this stuff, you can't. It's one of the modern classics, after all.
(Another fun fact: The studio involved in animating it went on to become the animation department of Studio Ghibli, whose work I'm sure you're all familiar with)