This is called the drumroll technique- from Pat Huntoon's Technique Junkies- an awesome newsletter- to subscribe www.patstamps.com
This particular technique is done with glossy cardstock. you scribble with your watercolour crayons, leaving some white space on the cardstock. You need a wet cloth beside you so you can keep your fingertips wet. You wet your fingertips then "drum" lightly onto the glossy cardstock where the crayon colours are until the lines from your scriblling are gone. Keep your finger tips moist- the amount of water you use will change the look of the "drumming". Then just allow it to dry. the more water you use the lighter the colour becomes. on This particular one- you can actually see the marks on my finger tips, if you use more water it changes the look. Very easy- Pat has awesome techniques from easy like this one to some that are more complicated. Enjoy