wwii axis reenactment forum

Morning all
I'm doing a sani impression at the end of the year and I'm thinking of a temporary change of helmet colour to white with red crosses.
Can anyone advise on paint that would stand up to a weekend of rain but then be removable without trashing the original field grey underneath?

Artist's gouache would fit the bill - looks like whitewash if brushed on roughly, but can be cleaned off with washing up liquid and gentle scrubbing. Will certainly last the weekend!