Stupid man had question about developer

I have been coloring my wife's hair for years, Usually she bounces between shades of blonde but went red a year ago. A few months ago she changes to a different, lighter reddish blonde (a clairol red/gold base). Now she wants to go lighter again but stay red (for the next couple months). Her hair is about 1-1/2" to 2" long and chipped -yea I know, very short and it's a pain to do the roots.

My question: I don't understand the different volumes of developers. I was told that the higher the more "lift" it gives. So if I am using a 20 volume now would 30 volume give me a lighter color? is that how it works?

Also, as a side question, when she does eventually go to a ashy, violet base blonde color are there any special things I need to be aware of?

Thanks for the advice!

Well it's a little more complicated then that and we can't give you a class on lifting & toning hair on this board.

Yes generally speaking, the higher the volume more lift can be achieved. But there are many other factors;
Your wifes natural color/base color.
Color/condition of previously colored hair.
& how much red pigment/from previously c/l hair.
What your mixing the developer with???
Timing?? on base
Timing?? on previously colored hair.
And lets assume you get it all properly lightened...dDepending on how much red/gold she pulls....Do we then need toning??? to achieve the desired ashy/violet blonde.