It's not that hard. Hair dye is a two step process. The first few minutes it removes color then it deposits color. That's why if you are redoing a color you put it on the roots then wait a few minutes until the color changes in the bottle to put it on the rest of your hair. If the instructions say 10 minutes or 20 minutes don't leave it on a minute longer or you'll get an ugly burnt looking color. Just look at "orange" heads that were supposed to be a tone of red or auburn.
Ash if for certain tones of blond. Most people will look green with it.
This might help people pick a color
Hair Color 101: Choosing the correct hair color
Hair coloring tips
Hair Coloring Tips For Coloring Hair Like A Professional!
If you do it the correct way there is no need for anyone to know you did it yourself.