(concept theory) Colors, How Do They Represent a Pony · 6:41pm Dec 27th, 2018
Now to be clear, I do not refer to individual mane and coat colors, but the shades and what they express.
Throughout the series, we have seen changes to a character's mane and coat color, mostly notably in 'Party of One,' 'The Cutie Chronicles,' 'The Return of Harmony,' and 'The Cutie Map.' These alterations have occurred in different manners, but do they share a connection?
Colrs & Cutie Marks
In 'The Cutie Chronicles,' it appears the Mane Six's manes and coats obtain a more vivid shade upon getting their cutie marks. This phenomenon is strange as it is questionable whether the cutie mark defines the pony or simply displays their nature and passions outwardly. I believe it may be both, the cutie mark completes them as their destiny becomes a certainty to them. Therefore, this color change is due to the cutie marks bringing out the pony's nature vibrantly. This may also indirectly explain why a pony would dullen in coloration upon having their cutie marks removed ('The Cutie Map').
Colors & Corruption
When Discord corrupts the Mane Six against their Elements in 'The Return of Harmony,' they became a monochromatic gray. This color change is probably different from the others in the list since the corruption was not only just against their Elements, but also their own being. The Mane six got the Elements of Harmony because they represented those traits, so Discord corrupted them against those relics by corrupted their own selves.
Colors & Elements of Harmony
It's rather strange, but if you look closely to the very moment in the second part of 'Friendship is Magic,' you will notice the Mane Six becoming even more vivid in their mane and coat's color shades upon earning the Elements of Harmony. As stated above, they gained the Elements since they represented those traits in their own lives. However, as other fans have speculated, did destiny also play a role in them bearing them? There is no confirmation of this, but if so, then maybe the Elements completed the Mane Six by clarifying their harmonious virtue.
Colors & Mood
This is more indecisive, but it seems mood can affect the vibrance of a pony's coloration, evident especially in the case of Pinkamena Diane Pie in 'Party of One.' This theory is rather self explanatory.
Unofficial Speculation:
What exactly happened to the clones in 'The Mean six?'
Recall how Chrysalis had taken a sample of DNA from each of the Mane Six and combined it with the photographs she had taken. Not only were the clones fakes, but they also were techically entities of corruption who did not match the real Mane Six's traits. It is questionable whether these clones, like those from the Mirror Pool, are alive and exact copies of the ponies they were cloned from. Regardless, the Elements of Harmony apparently detected this falsehood, and thus eliminated them as threats.