cheapo bottles of acrylic paint from The Works. At a quid a go, and with plenty of colours to choose from, they might not be "proper" modelling paint, but they do the job.
I tend to squirt paint out onto some plastic and then apply it neat with a wide brush. I do dip this in water every so often to thin the paint, but not much. It dries really fast and I like to give the surface a light sanding between each coat to flatten any wood grain that has risen.
3 coats seem to do the job, although I guess this depends on how deep a colour you require.
Before painting, I added some door hinges from scraps of wood. Afterwards, the varnished wood belt lines went on, making sure I cut a slot so it looked like the doors would open.
Talking of opening, brass handles from wire look nice - I bought some plastic mouldings but have lost the packet for the moment.
Each roof is a bit of plastic sheet pre-bend around a fat marker pen and then fixed with epoxy glue. I didn't use superglue as the fumes can fog the glazing. I had thought I'd paint the roofs grey, but they look nice white so I'll give them a light sand with a mild abrasive to matt the finish for the moment.