When I'm designing a character (pony or not), one of the first things I do is sketch out tons of different hairstyles for them. Just a basic round shape for their head with the hairstyle sketched over it. I try different combinations and styles from hairstyles I find online. I usually search for real life and anime ones, and of course pony ones if the character's an MLP OC. You can derive hairstyles from people you know too, yourself included.
One thing to keep in mind is the personality of your character too - for example, if you're looking for a hairstyle for an elegant, graceful character, a mohawk wouldn't really fit. I give calm characters straighter or 'smoother' hair, while energetic ones get rougher and 'spikier' hair. (Compare Fluttershy to Rainbow Dash, for instance.)
Drawing the hair from different angles is also really useful, it will help you know whether or not your hairstyle makes sense in 3D space. (ie does it still look right when viewed from behind or an angle, or does it look off?)
You can use the same process for tails too.
As for colors, I pick a color that is either close to the coat's on the color wheel (analogous color) or a color that is opposite on the color wheel (complimentary color). I usually make either the hair or the eyes a complimentary, and the other an analogous. That makes your character's hair/eyes 'pop' more.
Keep the value of the color in mind too, you usually don't want to make a character too dark or too bright, that makes it more difficult to do shading or lighting over them later on.
Anyway, that's how I go about drawing hair and choosing colors, I hope that was useful to you.