Painting
Although you dont need to paint polystyrene hive parts, like the wooden equivalent, they will last much longer if you do.
Masonry paints or even the cuprinol shades can be used.
Cleaning.
The hives are best cleaned with a solution of washing soda. This will dissolve propolis and clean off any dirt etc. Be careful trying to remove propolis and wax with the hive tool, as it will dig into the polystyrene if the tool is not kept flat.
Sterilisation of the hive can be carried out with a solution of household bleach, normally a 10% solution is recommended. Virkon S, obtainable from farm suppliers and some vets is also a very good cleaning liquid. Wear suitable protection, including eye protection, when cleaning your hive.