The volume of the developer indicates how many shades the hair dye will lighten your hair. A 20 volume developer can only make your hair 2 shades lighter, a 30 volume developer can only make your hair up to 3 shades lighter, and so on.
Color is categorized by numbers, 1 being the darkest and 10 the lightest (though it can go as light as 12).
1 - black, 2 - darkest brown, 3 - dark brown, 4 - medium brown, 5 - light brown, 6 - dark blonde, 7 - medium blonde, 8 - light blonde, 9 - lightest blonde, 10 - platinum blonde (almost white)
You say the color you chose was B51, the 1 indicates that it's an ash color (brown is the color of your hair, and ash is the reflection your hair will have). So in other words, 51 is a light ash brown. The B is probably used to help consumers see that it's a high-lift dye. Any number that comes after the first is simply indicating the reflection your hair will have (or at least should have).
1 - ash, 2 - silver, 3 - golden, 4 - copper, 5 - red violet, 6 - red
So, say you want a medium golden brown, you would opt for a 53. Of course the end result also depends on your starting haircolor. If your hair is black, it could be either a blue black or a red black. If you want a golden tone to it and your hair is blue black, you should stick to golden tones, however, if you don't want a golden effect and your hair is red black, then you should stick with ash colors.
You're on the right track, you picked a high-lift color to lighten your black hair. Your hair being black, it's a 1 and you want it to be a number 5. That means you would have to lift it 4 shades to get the desired result. I don't know what developer they provided in a box, but in order for you to go from 1 to 5 you need a 40 volume developer. Try using half the tube of the dye and all of the developer, you might be able to get a different result. If that doesn't work, try the B61. But you must remember, your hair will not lighten where it was previously colored, so you'll probably only see more results on the roots. Also, do a strand test first to see if you like the result. Hope this helps. Good luck. ^^
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