There are many variations on the recipes for slosh, but the key features are that it is soap based, and usually a foam.
I have also seen cornflour, glycerine and gelatine quoted as other useful additives, but haven't yet investigated these thoroughly. Glycerine (glycerol) however is commonly used as a foam stabiliser in cosmetics and is likely to do so in this application.
A further variation I have rarely seen is to dissolve soap flakes into boiling water with a small quantity of glycerine and food colour.
Key advantages of this type of mess are its quick clean up relative to gunge, natural opacity, relative low cost and ease of availability. These advantages see slosh persistently being used in the UK for theatrical use.
1 stick makes approximately 1 bucket full, so if other quantities are required the recipe will need to be adjusted.
At this stage add any colouring you want- children’s poster paint works well, but food colouring is also possible.
You will need to experiment with the quantity dependent on the stick and how much slosh you want to produce, but a good starting point is to use ½ to ¾ litre of water per stick. It is possible to top up with further water as you mix, but obviously virtually impossible to remove it.
If you want to use it immediately, then check that it is not too hot if you have used boiling water. If you are making up in advance you will find that the foam drains back relatively quickly (Within about 2 hours you will be back to the watery mix). You will therefore probably want to re-whisk just prior to use. You may also want to tip away any excess fluid as you see fit.
If left for a significant time (e.g. overnight you will find it very difficult to re-whisk to the original volume. Due to this, it is worthwhile making up the slosh relatively close to when you wish to use it.
Slosh away to your heart’s content!