Ok, I guess I'll hop in here

 I am a hair stylist and I'll tell you that color is the hardest skill to master!  I'll try to make this short and sweet with out being to technical.  Here goes.... Hair color is just like they taught you in art class primary colors, secondary colors, and so on... If you have no idea what you are doing then you can not go and grab a box of color off of the shelf and expect it to turn out exactly as you planned.  There is a lot of information on the box about the color itself that is confusing to people who don't know.  Like another poster was saying, there is a number and a letter or 2 on every box.  The number is the level the color is 1-12 (1 being the darkest and 12 being the lightest)  You have to know your own level in order to choose the right color you are trying to go.  Most box colors come with a 20 developer which will lift the hair up to 2 shades.  So if you are a level 5 which is a medium brown and you grab a box from the store with a 9NB on it, your hair will be darker than expected because you wanted to lift 4 shades and you are working with a devloper that will only lift 2 shades.  Next is the letter which is most important....it is the actual color, A is ash, G is gold, B is brown or blonde, N is neutral, R is red, ......and so on (too many to list)....you want to choose one that compliments your natural color. (going back to the color wheel) If you pull red or have red undertones then you should use an ash color...ash is green and green cancels out red.  If your hair doesn't pull red, and you use an ash color then your hair will be green.  G is gold and if you don't want a orangish gold tint to your hair than stay away from it.....and so on.  Semi-permanent color will not lighten your hair, only permanent color will.  Therefore semi-permanent is less damaging but it doesn't cover grey. So if you're looking for grey coverage then permanent color is what you want.  Also you can not go by the color you see on the box or the color swatch they have on the shelf.  If you look in the fine print, it says that the color shown or the swatch was dyed on white hair which has no undertones and will come out that exact color.  So unless you have white hair, you can't go by the swatch.  If you have really dark hair then you should pre lighten your hair first.  All of this is confusing and should really be left to the professionals.  Coloring your hair at home and messing it up, then going to a salon to have it fixed can cost mega bucks!  Most places corrective color starts at $85 and goes way up.  I know many people color their own hair and have much success but some of us aren't so lucky.  If you insist on coloring your own hair then please do as the directions say and do a strand test first!  Then you will know exactly what color your whole head will turn out

   Sorry this is so long!  Hope I helped.  If you have any more ?'s please ask!