Hair Crazy. I don't know if this is the best place or not, I just stumbled across it, and because I bought so many colours, I haven't needed to replenish them yet. I also grab the odd colour from a pharmacy, which is the "Fudge" brand that we have in Australia.
What you need:
Tinfoil (pre-torn into pieces, big enough to wrap your hair - it will depend how long your hair is)
Plate or palette
I always cover my plate in plastic wrap so I can just wrap it all up and throw away the excess dye at the end. The plate is clean too! So I don't need to worry about cleaning the dye off it. When I dye my hair, I try to wear an old top in case I spill dye on it. One that you can take off without going over your head is best - a zip or button up top. Finally, I place some plastic wrap on the bathroom countertop, so if I do spill dye, it is protected. The only issue is, I always spill dye everywhere where there isn't plastic wrap. It's called Murphy's Law.
I mix my colours on the plate, I never really use a colour 'out of the bottle', I love making my own. So get creative and mix your colours! Some knowledge of what might happen to the colour when you mix is useful... I wouldn't mix green and red, for example ;) You can see a post about what colours I mixed together here. If I want the colour to be more pastel, I mix in some conditioner with it. I actually mix a little conditioner into all of my colours, so my hair gets some extra conditioning love. Keep in mind, the more conditioner you add, the lighter the colour and the less time it will last. This time, I am going more vibrant and I know these colours wash out to a nice pastel over time.
I wash it out in the shower, gently pulling the foil out under running water. Then I wash it a couple of times with shampoo to get rid of all the excess dye. Finish with a good conditioner, and then... Admire your handiwork.
Easy? I have a few extra points.
1. ALWAYS test for allergies to a dye before you use it. There is generally a warning on the packet and instructions on how to do a test
2. Bright or pastel colours won't work unless your hair is very light to begin with
3. I don't recommend lightening your own hair unless you know what you're doing. I get my hairdresser to do mine, and you should too if you're unsure of the process, or how your hair will react.
4. Be creative! Mix your own colours, try different combinations and be yourself.
5. Remember, it washes out! But sometimes it can take months, depending on the colour and how 'grabby' your hair is. If you're looking for something less permanent try hair chalk as it washes out on the first wash.