Make up some buttercream using 175g of icing sugar and mix it with 75g of butter. Be sure to add the icing sugar a bit at a time so that it doesn't fly everywhere and make sure you sift it in so you don't get any lumps. Next add your flavouring, I used vanilla essence but again you could change it to whatever you want, orange essence or brown sugar work well, maple syrup is gorgeous if you have any, just try not to clash with the flavours you put in your cake. Add a little bit of milk and some green food colouring and mix well until light and fluffy.
Next use some ready made white icing and mix a little of it in a bowl with a little orange gel food colouring. Mix it really well so you don't have streaks running through it then repeat in another bowl with a tiny bit of green gel food colouring.