The importance of bases?? (pics included)

Hi, could someone explain to me what the importance of getting the right base for your desired colour is?  Before I joined this site I didn't even know that they existed/were significant and I still don't know which base to use for what.

For example, when I strip my hair of the black (which I will do soon) it will be a rainbow of white, blonde and orange.  What will I have to do to get a nice medium brown all over...bearing in mind my hair will be VEEERRY porous.?

My hair now:  Exremely dark, almost black reddy brown with bright red roots...eugh, disasterous!



I would like a colour similar to these, but if possible I would rather be a level 4 than a level 3...it just depends on how dark it will go after the stripping..

 
Coffee brown

 
Light Brown

 
Medium Brown

 
Iced brown

To acheive results like this would I have to use something like Light ash brown to counter the orange left on my hair? Like this???



Edited by aubergine

Thanks a lot, the tip about the uniforming the colour remover is particularly helpful.