The matter of taste.
If you are wearing makeup than probably you are woman, or a gay, who knows. But if you are wearing it, that means that you are doing it on purpose. With some intention in your mind. And you should ask yourself what do you want today? Is it to make your day brighter, or just make yourself lost in the crowd? Is it to impress someone, or push your friend forward in a meet?
On the other hand you could be a person who is thinking only what other will tell. And in that case your choice is pretty simple. Use appearance that will suit those people, whom you are used to listen, that's all. But you'll have no profit from such preference. It won't reflect your feelings and you'll be just fashionable, or even worse, you'll be looking ridiculous. In a black dress with green lips.
So what is the main purpose of make up? The answer simple as always. Whatever you'll choose. If you like to follow the crowd, follow it. If you are worrying what your classmates or coworkers will say, then ask them what you should wear. But don't be surprised if one day they say we don't care about you. And at last, if you like to follow your own mood and desires then you'll be the one, who is making the rules. And others will follow you.