color troubles

im almost done with beauty school and i know im skilled at many things one i just cant get is how to tone high lighted hair i turn it to where it has to much ash or to much red i just cant even it out. the most horrible part is i did my good friend the other day she wanted go from dark like a level 5 to blonde level 8 no problem did that. then came the bad part im pretty sure i had the right colors to tone with this time i knew it was going to work, then what did i do i used 20 vol developer instead of processing solution i wanted to cry for her it was like a grey green tint. all i no is i better get this down in a few months or ill no coloring for me but i know i will get there.....