I wanted to be able to set the pen colour and the sprite costume to the same colour.
After playing around , then reading the manual, then going for a lie-down for a rest, it turned out to reasonably simple in the end 
Key points to note:
makes sure all the visible pixels of the costume are turned to plain red (which corresponds to hue = 0)
controls the pen colour (valid hue values are from 0 to 100)
controls the colour of the sprite. (We have to use hue x 2 as Snap! colour block follows the Scratch convention of being in the range 0 - 200)
