One thing I noticed, was having that decision already made for me - "these colors are what I need" - allowed me a great deal of freedom while I worked. I wasn't pondering which blue to use, do I reach for my favorite color (periwinkle!) again, do I add black...I just grabbed some pastels and a few watercolor pencils, and went to work!
The top two drawings are on Wallis paper, the bottom is on a pastel board (masonite primed for pastel). I used only watercolor pencils on the top drawing, and the bottom two are a mix of pastel and watercolor.