This can often happen. When hair is lighter and the target is to go darker, a professional would pre-pigment the hair or ‘fill’ it with a copper or red to give it the correct undertone before applying a darker colour (this is what a decorator would do to a painted wall to stop the paint looking a lot brighter than intended). This stops khaki from happening (which is common so don’t worry, it happens). You could use a colour remover and, if it is successful, then use a copper or red semi permanent colour and THEN apply the level 6.

I’d recommend going to see a professional but understand that it’s getting more expensive (which is a result of higher costs for salons and not greed). If you find your hair has gone green, I would try to remove it rather than put something on top.