Working as a hairdresser for a few years, I saw some real colour horrors come into the salon. From ladies with khaki green hair, to brassy yellow/orange. Luckily it is easily fixable most of the time.
It all comes down to what undertone a hair colour has: warm or cool. An example of warm toned hair colour is warm chocolate brown, wine reds and coppers and honey blondes. A few examples of cool colours are blue-black hair, ash blondes and greys.
So what happened when you tried to go blonde and it came out khaki-coloured? Well, a few things: the darker your hair the more colour pigments you have and it ranges from blue (which is the easiest destroyed by bleach or lightening products) to red (which is a little more resistant) to yellow (which is the most resistant). That’s why if you didn’t lighten your hair sufficiently, it may come out yellow or a brassy orange because there is still an abundance of red/yellow pigment in the hair. Usually to counteract this brassy colour, a person might reach for the white-blonde or ash-blonde colour. That’s usually about the time they get khaki-coloured hair. Why? Because a yellow undertone (left in the hair) and a blue undertone (from the ash-blonde) makes green.
It all comes down to the colour wheel: choose the complimentary colour of your “mistake” and you will neutralize it. So if you get khaki-coloured hair, use a colour with a red undertone. Green and red neutralize each other. If you get brassy orange/yellow coloured hair, use something with a purple/blue undertone. But be warned! Usually 5 to 10 minutes left on the hair will suffice. An easy at-home remedy for the khaki hair horror: use ketchup or cranberry juice! Leave it on the hair for 5 – 10 minutes (while always keeping an eye on the results) and rinse. If your highlights are turning brassy, look out for products that contain purple or blue pigments. They even make bleach that has built-in blue pigments to neutralize the potential brassy yellow/orange.
So if you are ever in a pinch with hair colour just remember the colour wheel from primary school: go across the colour wheel and you neutralize.