I'm not sure how well it would work on foam, but Paperclay works well for me. It is air dry and it is naturally coloured, I believe (which is just an off-white). With some water, it should apply easily. Final product should be lightweight.
:: 2013 Events ::
AOD February 16-17 [Sunday]
FanimeCon May 24-27 [Full Weekend]
Kintoki-Con July 26-28 [Saturday]
:: 2013 Cosplay ::
Kagamine Rin [Lollipop Factory]