You need to go for a specific external masonry paint suitable for a rendered finish. One of the big manufacturer's like Dulux would be a good bet. Their range is called Weathershield. If you budget stretches go for a silicate paint like those produced by Keim Mineral Paints. This is a very long life product and should outlast the six year maintenance cycle for conventional paint may times. A good proportion of the cost of painting a house is labour and access so a paint tha costs twice as much will not double the cost of the job. It is difficult to give colour advice without seeing your house and its surroundings. If the windows have white frames I would avoid white painted render unles you want a stark modern look. Silvery grey would look good with white windows. I would recommend you buy some sample pots and paint about a square metre of card, prop it up against the house and step back to look at it. Bear in mind that it will look a bit more intense on a small area than a whole building. Good luck with the project.