(I used to be a salon manager...I am fully licensed.)
Not all Sally Beauty Supply places have ex-salon owners as staff. I have been to some (as a professional getting combs, developer, etc.) and they didn't know anything. I don't recommend them, simply because you don't want to be playing chemistry set, if you don't know what you are doing.
Most people don't know the difference between a level 4 or level 7 color, or between a 10 or 40 volume developer, and you don't want to find out the hard way.
If you are not a pro...I recommend sticking with store box color. This is because store color is meant for gray coverage, and SLIGHT color changes. What I mean by slight, is within 3-4 shades of your natural color. You can obviously add red tones, or whatever-but store color isn't meant to take you from black hair to platinum blonde. Any drastic changes should be done with a professional.
Here is my advice for the best results with box color:
~If you are a going for gray coverage, and are looking at brown shades, always choose a color on the box that looks a shade LIGHTER than what you actually want to go. If you want dark brown, don't buy the darkest brown they have-it will look flat black. When in doubt, choose a shade lighter than you think you want.
~If your hair is already colored, and you are going for the same/similar shade (touching up your roots and grays) don't put the color all over. The ends of the hair can get overprocessed, and "fried" this way. Put it on the root area, and only put the color through to the ends in the last 5 minutes. This will freshen the color everywhere, without frying it.
~Before you attempt a drastic color change, think about your skin tone, eyebrows, eyelashes, eye color, etc. Black hair dye on a woman who is a natural light blonde or redhead looks horrible in most cases. She doesn't have the dark eyebrows and lashes to carry it off. Usually only brunettes can carry off black well...
~The difference between $3 store color, and $10 store color is usually the conditioners and gentler ingredients. The higher priced are usually gentler on the hair, and are less damaging.
~If your hair is getting very gray, stay away from super dark browns, and bright reds-unless you go to a salon. Gray hairs don't absorb the color as well, and a gray haired person going bright red, can get "pink" instead, etc. because they don't have the base tones in their hair already. This is why many senior women have color their hair light brown to blonde.
~If you color, use shampoo and conditioner for color treated hair. It will keep the color truer longer, help it from fading out so much.
~Remember that a store color is meant to keep you within 2-3 levels of your natural hair color. They only have a moderate developer in them. Basically, hair has shades from level 1 to 10. This doesn't mean the "tone" (reddish, goldish, ash) but rather, the lightness or darkness from black to platinum blonde. Don't try to go more than 2-3 shades lighter or darker at home. If you have ever seen a woman who is obviously a brunette, and her hair is the "light orange" this is from using store color to try to go blonde. I guarantee that Velveeta wasn't the color she was going for.
The box color isn't strong enough to take a dark brown to platinum blonde.
If you have any questions...feel free to ask.
Anyways, I tried Clairol Natural Instincts semi-permanent color & I LOVE it. My hair is super shiny, the color is the best/most natural looking I've ever had & I don't have to bother with the tell-tale roots. Now when it's starting to fade the greys look more like highlights. Can't wait much longer than six weeks tho'.
) As soon as my purple fades, I'm going to dye my hair a shade slightly darker than my natural color (light mousy brown.. bleh) with Garnier. My poor hair has pretty much every color you can think of, from black to blonde to purple to purple-ish brown to dark red, you name it, and I've used all different brands but Garnier is my favorite. 
Great tips!
