Assuming the wall is clean and sound we can recommend Wethertex AP77 Flexible Smooth Masonry Paint

This is available in many Ral and BS colours.

If there is any algae or fungicidal growth we can recommend the use of Wethertex C10 Multi-Surface Fungicidal Wash prior to painting.

If the wall has an unstable surface we would recommend the use of Wethertex ST1 Quick Dry Stabilising Primer prior to Wethertex AP77 Flexible Smooth Masonry Paint.