Here’s my little trick for depicting folded cloth. I create a flat or rack brush first with some texture in it to make it look less digital, and in the Natural Brush Media window on the shape Dynamics tab I set Angle Jitter to Pen Pressure. This makes my brush react a little strangely and it takes a while to get used to, but it does help me paint cloth realistically.
I place a new Overlay, Multiply or Hard Light layer above my art and start painting in colour. Sometimes I select the area I want to colour using the Lasso tool, copy it on to a new layer and try different colours using the Hue/Saturation tool. I often start colouring the background because this will determine the colours of my bounce lights and shadows in the foreground.