Can I use henna (Lush) over regular dye?

[MedusaIsHavingABadHairDay]

I have light/mid brown, long hair with a few sprinkled greys (attractive huh) and have had it regularly coloured with chemical colourants for ages. But they don't last.. 2 weeks and it lightens and I can see the greys again.
I used to henna my hair years ago and I know the greys will still come through but I love the richness of henna.My last 'all over' colour was a only a few weeks ago and is already lightening and I'd like to go back to henna..

Is it ok to do that? Or will my hair melt or turn green or something!?

It should be fine but play it safe by doing a test bit. You can henna over dye and dye over henna as long as you can't using bleach (reacts badly!) although if you're trying to cover greys it may be worth using a body art quality henna which you can buy online as the pigments are stronger so it should last longer. You can also use Indigo if you wan more of a brown than orange colour. If you do a search, there are some forums all about henna for hair with some good advice.