Set your pen color to RGB or Hex! Just use the custom blocks found in the project. :) Want to set the pen color to HSL? Check out @DadOfMrLog's project! http://scratch.mit.edu/projects/12713728/
Thanks to @MathWizz for the RGB equation, and this website for showing how to convert hex to RGB: http://gristle.tripod.com/hexconv.html