It all depends on your natural hair colour. If it's black, no matter how "temporary" it says it is, it will not wash out easily, due to the violet tints to make the black dyes. Go for the lightest black shade they have, and do some test strands(you will most likely lose a bottle doing so). Get cheap kids barrettes with different colours and make a notebook with colours and how long you left in with which colour. tie and pin the bulk of your hair up, then do test strands close to the scalp(so the bulk will hide your tests). Time the sections five ten, fifteen, and the maximum time they have you leave it in for, then wash out and compare. Circle your perfect shade time and process your hair a week later(gives the test bits time to relax).
If not black, then go for a shade to two shades above your natural hair colour, and test to see how you like it. You can always go darker later, but going lighter is a PitA!
I dyed my hair after the sun bleached it from black to auburn red one summer(I blame heavy chlorine in the pools). I tried also to cover the white streaks that have always been in my hair(since I was about four or five), but I have found they will not take a dye. I even went to a professional to have her do it, and the white did not take a dye.
Because my hair is naturally that dark blue black, and has some auburn lowlights in it, I got Clairol nice n easy wash out dye in Natural Black, and processed it for the maximum time to get the same effect as five minutes with Blue Black.
My sisters friend is a stylist, and told me the above information on black dye, and when I called Clairol later about the nice n easy, they said the same thing. I actually helped a friend of mine give herself some black streaks in her red hair with the washout, and they were there until she cut her hair a year later(she washes her hair daily, and twice daily if swimming).
Oh, I also recommend getting two boxes more than you need of the dye(to make sure you have enough--my hair is thick, coarse, and down to my waist, so I need three to four boxes. This prevents you not having enough to finish.). I also kept the extra boxes for root control. Usually, if your hair is drastically different in colour, you need to touch up every four weeks.
Invest in cheap alligator clips(to hold the top layers until you are ready to apply dye)and long wide tooth throwaway combs(you generally can find cheap ones at Walmart). It's good for separating sections so you can apply the dye. Box of latex gloves(those plastic gloves were always so flimsy for me) if you plan on doing it often, and a box of shower caps(not all companies come with them). A large black towel because it's good for bad hair days...;p Junky shirt and pants to avoid ruining clothing from accidental spillage. If you have lady friends that dye their hair, you can have a "dye party". I help my friend colour her hair, and afterward, we have pizza and chick flicks(the menfolk keep an eye on the kiddos, or take them to the Playplace for a special treat--yes, my kid only goes to McD's for a treat). I don't colour mine anymore, as I donate, and also the dye doesn't take on what I wanted to use it for.
Hope this helps.