hi, i know this may sound silly but how long will a sponge cake keep for, i need to make a union jack sponge cake for friday but the icing is a bit difficult and some parts are complicated so i want to make a start on it, i always make a sponge the day before it is needed but this cake is very big for a street party and i am a bit unsure if i can get the colours right so i need plenty of time to practice as it were any suggestions would be great, also how can i get my icing dark red and dark blue as in union jack colours thankyou