Technically the color purple is called violet.( I'm only mentioning this because if you come across a color wheel you won't find purple on it) To get what we call purple, you mix red and blue, with a little more blue in the mixture. Start slow and mix your colors in small amounts until you get the shade of purple you want.
Basically, dying your hair is like painting a portrait. If you mix blue with red, it will become purple, but the more red or blue that you use, the more red or blue it will become. So if you mix purple with black, it will be black, which is why it is recommended to pre-lighten hair if it's really dark.