Custard is probably the most popular foodstuff used to get messy. The various consistencies can be altered to the user's liking and sometimes taste. The usual yellow colour makes it stimulating to the eye and fun to watch others wallow in it if you like the colour yellow. Due to its viscosity, it can only be spread so far so a proper full-body covering needs a few good litres of the stuff to prove sufficient, again, depending on the user's preference. It's also pretty cheap and easy to get hold of.
Thick custard runs slowly over the person it's poured over--covering them without all running off completely.
Custard can be either made up from powder, or bought ready-to-go in cartons at supermarkets. I would encourage the second option, as it removes the risk of getting it wrong and ending up with a thin soup of lumps, and also you don't have to wait for it to cool. One other tip is that if you have suitable containers, pour the custard into jugs or buckets for use. Cartons can be tricky to open, handle and get a decent pour from when you've already got messy hands.
Custard, like all dairy-based substances, may leave a lingering smell on you and your WAM room for a while, and also an oily feeling on skin. It's also something you'll need to check has been thoroughly removed from ears if you plan on concealing your messy fun afterwards!