If the pots are terracotta, wash and dry them thoroughly. When completely dry, give them 2 coats of ordinary household emulsion paint and leave to dry. In a shallow dish (or plastic tub) pour in about 1 or 2 inches of warm water and dribble in a few spots of nail varnish. Give it a swirl round with a cocktail stick to break up the spots of nail varnish. Stand the painted plant pot in the mixture and tip/tilt it so that the nail varnish adheres to where you want it to be. Stand and leave to dry. You'll get something like this.....