Hair Coloring Tips--Store Bought or DIY?

In My Humble Opinion
1

A little background:

I’m forty three and have been coloring my naturally dark brown hair various shades of red, due to increasing gray hairs. I’ve always purchased hair color kits from drugstores because it’s fairly brainless and I’ve always gotten good results. I use Feria color and my hair looks great. Lately, I’ve been looking at the L’oreal display “Colour Artiste”(name ?) where the color and a harmonizing high-lighting color are packaged together. I have some questions from dopers who put together their own kits from scratch:
[list=1]
[li]1) What is the advantage of mixing your own colors? It looks like almost all shades are represented in the drugstore display?[/li]
[li]2) Can you mix shades from different companies together?[/li]
[li]3) Is there a lot of chemistry to remember and a lot of measuring to do?[/li]
[li]4) Can you color your hair and highlight in the same session?[/li]
[li]5) What are your tips to keeping your color bright and your hair healthy?[/li][/list=1]
If anyone can answer questions I didn’t think to ask, please feel free to add your comments. Thanks for your insights.

2

Ah, come on dopers. Somebody’s gotta be dying to show off her or his knowledge.

3

Okay, I’ll bite.

Plain old dying your hair is easy enough. Just get the people at the store to put you together the ingredients and follow directions. It saves a little money and you can mix up new shades (though I always kept to the same brand of color to be safe). There’s hardly any measuring and no ‘chemistry’ beyond shaking to mix.
I’m not brave enough to highlight my own hair-- there’s too much of this curly mop to hope I could do it properly. The sun turns it light on it’s own anyway.
Oh yes, keeping the color bright? Hmmm… I use this purply junk that is supposed to keep my blondish color from getting brassy. And leave in conditioner is my friend, yes it is.