Greenish tinge after colour b4

Wait with bleaching until it feels healthy.
You can tone with a very diluted semi, find out the opposite colour (I think green - orange or red) (you can put a picture and invert colours in paint to find out the opposite colour)
Put just a tiny bit of it into lots of conditioner and leave it in for as long as you'd like.

Thanks for the reply, so would using 'touch of silver' or purple based toners make it more greeny grey? I only ask as I have some sitting on the bathroom shelf looking at me.

I would give the colour remover another shot, it is likely the green is left over from the brown dye. Putting a violet toner (like Touch of Silver) over green is likely to turn it blue, if your base still has yellow tones this could actually make it greener. Silver toners are actually blue, so if your base and the green staining are both pale enough you could get a silver shade. Otherwise you would be better off using a very diluted pink/red. Try a strand test with the Touch of Silver.
A pic would really help too.

I agree with wicked pixie, try every colour-remover/stripper you can possibly get your hands on, use every non-damaging product that exists in the whole universe before you decide to go for bleach - it should be your last resort

Apart from colour removers, you can deep condition your hair with certain reconstructing products like joico kpak or coconut oil, argan oil etc. Hair oils will make the dye bleed like crazy so give that a go