To apply colours for exterior walls, you first need to prepare the surface. If you are using a fresh coat of paint, wash the wall with soap and water to remove any dirt and dust. Then, wipe the wall dry with a clean cloth. Next, apply primer and allow it to dry completely before applying your topcoat of paint. Finally, use a roller or brush to apply your topcoat of colour evenly over the wall surface in light coats until you have achieved your desired result.