Hi Faisal
I have been experimenting with using a very basic palette lately. My basic palette is Titanium White, Cad Yellow, Cad Red, Ultramarine Blue and Raw Umber. Those seem to cover a very wide range of colour. I do not use medium in the bottom or mid layers. Or, excuse me, I DO add a little medium and thinner together to help bind the base layer of paint. So I can paint "thin" but not dilute the paint so much that it 'falls apart', I add a little clear medium.
I only use medium otherwise on any layer after the beginning layers. hmm, hope that made sense to read...