If its brand new pebbledash, then it might be advisable to put a stabilizer on it first, this is a clear liquid, and goes on very easy, its like water, and makes it easier to paint afterwards, plus holds the paint.
If its been previously painted, you will have no problems, like CID says, use a good quality paint.
And if it has not been previously painted and the house is pre-1914, don't!
If it goes wrong and you get damp in behind the paint (which will happen through the finest of cracks) it won't get out. And you will have a damp problem.
Personally I think pebbledash au naturel looks best; yes it can look a bit grey but so does paint after 5 years. And another 5 years after you re-paint. etc...
1914? Simply that before that, many walls are solid rather than with a cavity so any damp problem caused on the outside will get you on the inside. With a cavity wall, this is unlikely. And also earlier bricks were softer in most areas so that getting rid of problem coatings (paints, renders, pebbledash etc) is harder to do without damaging the bricks.
We had our pebbledash (fairly old, I think) painted professionally earlier this year. Decorator used Dulux paint and a brush as he said a roller wouldn't be any good for getting into all the nooks and crannies. Mostly it needed two coats of paint, but odd bits needed a third coat. We were very pleased with how it turned out and are planning to have the back of the house painted to match soon.
If you paint your house with a sealant paint, then what you are actually doing is 'tanking' the mortar or turning it into a long sort of bucket that can fill with water if there is a breach in the surface. This can actually drive the rain into your house rather than away from it.
However, the main problem arises if the render/pebbledash or whatever gets damp and that damp cannot escape. During a cold winter night that damp will freeze and expand - and 'blow' the pebbledash off the walls. You then end up with an even bigger problem.
Make sure that the paint you use is 'breathable'. It's technical term is 'microporous'. What it means is that the large water molecule cannot get through the surface into the render/pebbledash, however, the small water-vapour molecule is small enough to pass out of the render/pebbledash and back into the atmosphere - so the wall dries out (regardless of where the water got in - usually a leaky gutter.).
