Well this is one answer, but not a particularly helpful one I am afraid.
Can you find the piece that has chipped off?
If you can then I believe the colour can be accurately matched by many DIY stores as they have colour matching facilities.
If you cannot find the small piece then approaching the manufacturers should give you the code for the colour paint that they use - if it is an enamel paint you MIGHT be able to match it from the range that Humbrol do for model makers and that are sold through model shops - but that may not be as precise as the manufacture's code.
I hope that you can identify it easily.
Any typo that I make means the errorists win.
Any comments above are only MY OPINION and should be read as that.
That is actually quite helpful. The cooker will have a manufacturer's name, so will check that and find out the colour code. It is a much used green, well, around ten years ago, so there should still be some about.
AGA cookers do a small enamel patch kit of different colours, have used the black following the instructions it could almost have disappeared they have green but probably not a match for you but just making you aware that a product does exist
A dab of green Hammerite will make it perfect , but get anything on it mean time, even a blob of nail varnish, but remove it before painting, Acetate (nail varnish remover) will be perfect for cleaning it before applying the new finish anyway.
It's a shame that vitreous enamelling isn't as popular these days, as thats what the green really paint is.