Vocabulary: Hue means any shade or tint of a color; Tint means a lighter hue of a color; Shade means a darker hue of a color. 

Earth ponies: The coat colors of most purebred earth ponies consists of mostly warm colors, such as red, orange, yellow, and brown, and various hues of green. The same applies to their eye colors. Their manes are usually straight or a little wavy, and as the ponies get older, their manes may turn gray faster. Luckily, their kin lives the longest out of Unicorns and Pegasi.

Pegasi: The coat colors of a purebred pegasus mostly consist of tints of cool colors such as blue and green. Meanwhile, their usually soft and curly mane colors may include tints of warm colors such as pink and yellow. The eyes of pegasi, however, can be nearly any neon color of the rainbow.

Unicorns: The usual coat color of these majestic ponies include any hue of white, pink, or purple. Their straight or wavy manes may prefer shades of black, blue, pink or purple. Their eyes are also various hues of blue, pink or purple, but mostly shades of said hues.

Similarities: All three of the ponies' coat, mane and eye colors intended to camouflage themselves in their original habitats. Earth ponies mingle on the land and amongst the trees, pegasi mingle in the air and amongst the clouds, and unicorns mingle amongst crystals and... nobility? Unicorns are just too majestic!

Bonus Headcanon: To pick a side on whether or not Pinkie Pie is related to the Apple family, I think she is... despite being mixed race. First things first, the Apple family is believed to be consisted of purebred earth ponies. Since I theorized that most earth ponies' coat and eye colors include orange/brown and that their manes gray faster, all that may sound familiar to Pinkie's father, Ingenious Rock. Unfortunately, his brown coat didn't get passed down to his daughters, but his purebred genes must've been so strong that said daughters turned out to be earth ponies as well. Pinkie's mother, Cloudy Quartz, on the other hoof, looks and sounds suspiciously similar to a pegasus. That's because I think that one of her parents must've been a pegasus themselves. As a result of having foals with an earth pony, Cloudy Quartz was born without wings. Further "proof" that Pinkie Pie is mixed race is the "fact" that Soarin is her cousin. Whether or not Pinkie is a third cousin twice removed from the Apple family, her close family is pretty confusing as well, and more secrets may be yet to be revealed...

Thanks for reading! I'll answer to any questions, suggestions or other feedback on my coat, mane and eye colors headcanons/theories the best I could.