I reckon you will need to add yellow to get green, the icing will only have a very pale yellow tint. And my DH requests cakes too: last year it was in the shape of the arc de triomphe as we'd just got back from Paris and the year before it was a rocket too!
Don't forget to put sparklers on around the base so when they are lit it looks like the rocket is taking off!
I used butter cream when I made Abbey's cake, because I wanted pale pink and not a salmon colour, as it would have turned out, I substituted half of the butter from copha. I found a recipe that suggested using all copha, but I just couldn't do that. LOL. So half and half it was, the colour turned out great (much paler than butter cream) and the taste was yummy too!
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