It depends on the type of paint but it should be ok imo ~ check the spray can itself and they will usually state the suitable uses for it. E.g. Metal, plastics etc.
The other option if you're not sure is to get a piece of plastic from the kit that isn't used like from the plastic parts tree. Spray that with a few coats, lightly at first, building up the layers ~ then when it's completely dry check to see how well the paint has adhered to the surface.
Fyi ~ "Humbrol" actually make model paint in "Azure Blue" but it's usually in paint pots rather than spray cans ~ obviously no good for doing the car body.