What is the cheapest option and best paint colour to use over smoke damage walls?
Paint color has nothing to do with covering the smoke damage. Any color can be used. It will take 2 or more coats depending on the amount of smoke you need to cover and if a primer is used.
The most important thing to remember is the deposited smoke particles need to be removed through cleaning and the surface sealed with a good primer before painting. If this isn't done the smoke damage couple bleed through the paint, making stains, and cause a loss of adhesion, peeling.