Here are my additional tips for home hair colouring (I have trust issues with hair dressers colouring hair, especially in asia, and grew up in a family of females so am well versed through experiences good and bad in the DIY version!

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- colour from the back/bottom up. Have all your hair twisted on top of your hair, and slowly section your hair off horizontally as you work your way up. This is the easiest way for me to make sure I'm getting all the roots in my thick hair. Some will prefer making sections and clipping them up separately before you start colouring, but I find a single clip on top that you undo to let another section of hair out and then re-clip is the easiest way to work from the bottom up. You need to find whichever works best for you of course
- Be methodical and work systematically; the aim is to avoid what I think of as leopard spotting, where the dye has been applied unevenly or even missed a small patch and so you get darker and lighter patches. Colour variation works in streaks but not in blobs!
- while the colour is on your hair developing, wrap your hair in a (clean, new) plastic bag. Start from the back to capture all your hair, then twist the bag (tight (but not too tight) at your forehead to keep it secure and put a clothes peg on the twist. This makes it much safer to go watch TV while it's developing; you wont get hair dye on the couch.
- if you don't have a low neck (back and front) top kept especially for hair dying, the safest way to go as others have posted is waist up nada; why risk spoiling clothes if you don't have to.
- and my no. 1 tip for first time dyers with long or thick hair - buy two boxes. It's still pots cheaper than going to a hair dresser, and it makes it so much easier than trying to spread a smaller amount evenly over alot of hair. If you don't need the second box, it'll keep til next time. The boxes will usually recommend long hair use two boxes anyway.
Have fun with it! It's really not as hard as it can sound, you just need to have everything to hand before you start, keep an eye on the clock when th colour is developing, and be eagle eye methodical when applying.

I also recommend Feria.