How do you ice a cake with several colours?

[StoneBaby]

I'm doing a Noddy's car cake for DS's birthday. The recipe uses buttercream coloured yellow and marzipan coloured in red and chocolate for the other parts.
We don't like buttercream so I'm looking at what to use to cover the cake and get the yellow.
I'm thinking of icing the cake in yellow and use the coloured marzipan for the other parts but DH would like the cake to be fully iced My icing skills are not the best and I wonder how I will use various colour icing and it not 'fusing' with each others!
How will you do this icing?

I haven't made a birthday cake for ages but when I did, I always coloured ready made white fondant icing with food colouring.

I've just looked on My Supermarket and see that Renshaws now do a pack of coloured icing which has 4 x 100g strips coloured red, green , blue and yellow. I don't think 100g would be enough to cover the whole car so in your shoes I'd buy a pack of white fondant icing and a bottle of yellow food colouring and knead it in a couple of drops at a time until you get the depth of yellow you require.