The developer is typically hydrogen peroxide. It is the oxidizing agent used to open up your cuticles so that the color dye may be deposited. Developers can come in a creme or "clear" formula. The creme formula has conditioner and the clear doesn't. They serve the same function.
Developers come in different strengths known as "volume." The volume starts at 10 and ends at 50. In a typical drug store box of hair dye, they include a 10 or 20 volume developer. 10 is used to add a tint of color and 20 is used to lift the hair color by one or two shades. Sometimes bleach is added to developers in order to dramatically lift the color of the hair.
This is the pigment that will be deposited in your hair.
Choosing a color
Just look at the picture on the box, right? This has never worked out for me. First, you will want anything with ashe in the title, because the blue tones in the dye will prevent your hair from turning orange. Avoid words like "warm" or "golden." Secondly, get the lightest shade of blonde you can find. Black hair is very difficult to lift, so don't worry about it turning out too light. The lady in the picture of my box was practically albino, and my hair is barely medium brown.
This is the corrective color and people usually use it to correct brassiness (orange tones) in their hair. You can mix the toner (which is a special kind of hair dye) with a developer and apply it as you would hair dye. Toners come in different colors, so you have to see how the dye turned out in order to pick the correct color.
I was all over the place with this process, so I'm just going to show you what I used and how I did it.
You can see that I have a my developer, a box of hair dye and a tube of hair color. I first mixed the developer with the tube of color and found that it wasn't light enough.
I then bought a box of drug store hair dye. I ditched their developer (which was in the white narrow top bottle) and mixed my developer with their dye. I used a 1 to 1 ratio. I'm not sure if this is correct, but it worked for me.
I kept the first dye in my hair for an hour with not much lightening. I kept the 2nd dye in for about 35 minutes. I did a strand test first, so I'd recommend doing this to see how long you should keep the dye in your hair.
The first one is a typical developer that you can find at Sally's Beauty Supply. I got it for about $2. The second one came in a bleaching kit. (I tried to give myself highlights, chickened out.) The bleaching kit came with 40 volume developer. Both were clear.
After the initial dye, redye and all of 10 minutes of bleaching highlights, my hair came out to be a medium brown. It's a bit more red than I like and I haven't had a chance to fix it with toner. I'm giving my hair a rest for now and will update if I do change it again.