i used black food colouring to make a grey sugar paste, the following day the colour had turned a pale green, and i don`t know why?  I also watered down some black for my airbrush, and the same thing happened , does anyone know why?

Sounds like something is wrong with the black food coloring that you have. It may be old. Is it liquid or the paste kind? What is the brand name that  you are using? I have black paste and it turns grey or black depending on how much I put in. It turns blacker the longer it sits in refrigerator. What do any of you experts out there think?

AThe Wilton gel colour has green undertones. I really notice it when I wash my hands or utensils after using it. I only use it for making black modelling paste for figures or black royal icing.

If I want to make grey sugar paste/fondant I add ready made black fondant to white fondant in the desired amount. That gives a true grey in my opinion.

AI learned this tip here. To make grey that does not turn green or purple overtime, mix light/sky blue and a little black. It will give you a really nice grey :)

Yes, it's the paste color used.  That often happens with blacks and some brands are worse than others :(

Here's another tip for making black:  When you finish any decorating project and you have say a tablespoon or two of icing left in the bag - scrape it out & save it - and continue collecting *any and ALL* colors together until you need some black.  I keep it in the fzr.  Mix up that mixture of colors well - yes it will be an awful color but once you all some black paste/gel it won't take much to get a nice dark black which won't have that bitter taste more black icing have :)

Another hint: many people use chocolate icing instead of black.  A nice dark brown will look black once it sits on the cake/is exposed to air.  Again, keep any unused in the fzr (well marked so you remember what it is).