I have dark brown hair and want to dye the bottom half a dark purple. I've read how to put it in and dye it, but when washing my hair right after I dye the bottom half, do I wash all of my hair with it, or will it mess up the rest of my hair?

I would suggest just rinsing out the color with some cold water, always use cold water when you are doing a color like that or else the color will fade quicker, and than after getting most of the color out shampoo your hair, and condition it. It shouldn't mess up your hair as long as you get all of the color out but it's always better to be safe. Hope it turns out good! :)