I been wanting to do this for a while! It took me 2 tries. I have Asian black hair, which I now know takes forever to dye. Initially, I used the one for darkest hair. It did almost nothing, and I let it sit for an hour! The lightest it turned was really dark brown. My friends didn't even know that I dyed my hair!
I was really disappointed. I wasn't going to try again but all I saw so many instagram, twitter and blog posts with people who had so much success! I was determined. So, I went back go Walmart and got the the lightest shade: the one for blonde hair, and it worked!!!
If you have hair similar to mine, I suggest getting the lightest color. It took less than 30 minutes for it to turn a lighter color. But be careful! You have to really watch your hair; it turns color fast!
Update: Do not apply the color linearly. Try applying in a zig zag pattern. By applying it in a staggered way makes the ombre effect more natural. There are parts of my hair where you can see a line of my natural hair and my colored hair. (Thankfully it's under my hair).
Watch out for baby hair. It dyes a lot faster than your normal hair. This may sound obvious, but I thought I say it just in case. When I did this, I pulled back my hair into a ponytail and sectioned it. (Don't worry, I have a point to this.) I started applying from the back to the front. Well, most of my hair in the back is thinner than the rest. When I was waiting for it to dye, the back of my hair was lighter before the rest of my hair.
Okay, that's all I got. If you try to do this, I hope it goes well for you! I'm really happy the result! What suggestions do you have someone dying their hair for the first time?