Presumably you're using liquid icing? In which case, sorry, it's not going to get dark enough. You might fnd that adding blue helps get the right shade, but it won't be dark. You need paste colours for that.
Have you an alternative? What are you trying to do?
thanks girls! I'm so annoyed-we were at the cake shop and went for liquid instead of paste! I'll go to the shops now. Darrel Rivers-Thankyou for your kind offer. where do you live? Nowhere near Twickenham I guess ?
If you're desparate, you could buy some of those icing writing kits. The green in that is a bit too bright for Ben 10 but it's deeper than you'll get with liquid. Good luck! If it's any consolation, I just had a very similar experience trying to get blue for a Superman cake. I couldn't get it deep enough (by a long shot) but none of the boys seemed to notice (8 y.o.), they still loved it.
I guess my desperacy is to get the cake ready and the icing set by 4 pm today! we had the same thing with our dinosaur cake last year-ended up with a mint green dinosaur-but you're right-the kids loved it!
The day has been saved! A lady that runs her own professional cake company locally kindly gave me some green paste to save me trekking all the way to the shops. Thankyou for all your help ! Have so many tips now for next time!