Hello Joneskay!
A White paper is a presentation of a problem and the solution. Basically to highlight the features of the solution, product and service. Easily said: A project/product presentation. A yellow paper however, is a more formal approach to the technical specification of the project. Easily said: A technical specification of the project/product.
Many ICO-projects have a very shallow white paper, not really specifying anything else than vaporware. For us it is important to be as transparent as possible with our investors (hence the white paper+yellow paper) as we have a finished working prototype.