Freelance colorist Andreas Brueckl has some great tutorials for colour grading using Assimilate Scratch. This whirlwind tutorial reveals the process he used for a TV commercial which involved a lot of complex alpha matte based grades. This tutorial gives you some idea of the power and simplicity that Scratch can provide.
Assimilate Scratch Colour Grading Tutorials
Andreas also has a more in-depth tutorial on creating a monochromatic look in Scratch.