Hi, I'm Carol

Tons of basic info in this post:  http://talk.hairboutique.com/forum_posts.asp?TID=40328
It is Claude's sticky on top of the Hair Color forum here.

Color fix is a low peroxide thing that removes dye, usually leaving hair yellow or orange (whichever color the peroxide in the dye turned it to when it was dyed), at which point it can then be dyed over.

Fillers - I may not know everything about them, but they are generally used before hair is dyed, especially on hair that was bleached a light blonde, to help keep the bases in the darker dyes from showing up and making a dye that you would put over lightened hair come out more true to color.

Yes, DON'T dye your hair lighter over previously dark dyed hair, color won't lift color and it won't do anything but help damage your hair (I believe Claude's link explains more on that).

They may give you advice on color at Sally, it just depends who is working that day if it will be good advice or not!  Sometimes it is!  If you want to use box color from a drugstore, calling them is a good idea, they don't write what bases are in those colors (ash could mean green, and it could mean violet...you need to know before you use it because it could be good or bad depending on what you've got...so, they are really the only ones who'll know whats in the color and be able to tell you what to use based on what you have and what you want).

Don't worry about asking repeat questions, its nice if you look at the old posts to help keep people from repeating themselves, but not everyone does, so I think most of us here just don't come here on days when we don't feel like repeating ourselves!  If we are here, we probably don't mind.
Good luck!