One of the reasons that many amateur paint jobs end up looking dull or listless once they are dry is that the painter fails to use a primer before applying the first coat of paint.
Primer is actually one of the most important parts of the process. It keeps stains or imperfections from the deeper layers of the wall from showing through your paint, helps your paint to spread evenly to the wall without significant blisters and increases the time between paint jobs. It also helps the layer of paint appear much more bright and vibrant.
If you are painting with a lighter color over a darker color, one of the ways that you can make sure you hide the underlying color is by mixing a small amount of the paint with the primer before applying it. This is only possible if both your paint and primer are of the same type (latex or oil). You do not need much paint in the mix but you should be able to tell that the primer is shifting towards the paint color.