Can I paint over XPS insulation?
Dow, a manufacturer of XPS products, on their India web site, does not recommend using XPS under a roof deck.  This is due to the possibility of condensation occurring between the insulation and the structure.

As far as painting goes, US codes require a 15 minute thermal barrier over an interior foam, in the event of fire.  This could be satisfied by gypsum wallboard, obviating the need to paint.  For noninsulation interior uses, Dow permits latex paint. 

Instead, they recommend using an "inverted roof" where the panels are placed OVER the waterproofing and covered with a ballast material that will weigh the panel down and protect it from sunlight.