I brought 2 packs at £6 each and this has made the whole helmet with some to spare incase i have a mishap and any color will do, whatever suits you.
I the rolled out the clay to a decent depth and started to layer it onto the head. Don't be too sparing with it, remember, this is the basic support structure to everything you're going to add after so it has to be strong.
You can blend it in by dipping your fingers in a bowl of water and smoothing the edges to merge.
You have no need at this stage to be neat, really whack it on and build up the important parts and always make sure its symmetrical. Work the clay smooth with wet fingers until its a rough shape of Ironmans helmet.