First time decorating a cake. Sonic.

Birthday cake I made today. It was 2 8inch Sponges with cream and strawberry jam in the middle and buttercream icing on top.
The person I made it for loves sonic so I had an attempt to try and start decorating cakes.

I think if I did it again :
I would only use one cake because the illustration was a bit weird high up.
I wouldn't use buttercream instead I would use coloured white chocolate or ready rolled fondant because even though I used a whole bottle in this experiment it didn't turn blue enough (I watched YouTube videos telling me how to make white buttercream but it didn't turn white at all). Can you colour buttercream brightly ? Internet told me I could, I failed though. Does anyone know where you get white food colouring from in the UK?