The Sooty gene is a modifying gene that acts on horses coats to darken the upper areas of their bodies. It can affect bays, chestnuts, palominos, buckskins and many other colours. This is how I render Sooty horses on Gimp, with Blossom acting as my lovely assistant XD
Step 1. Choose your lineart. I always add an Alpha Channel to my linearts so that I can make the background transparent. If you don't know how to do this, I explain it in detail in my tutorial on adding backgrounds to your images here:
http://www.animationsource.org/board/how-to-add-backgrounds-in-gimp-t35394.html
I then add a new layer and set it to Multiply.
Step 2. Using the lightest colour of my horse's coat, I apply an even layer over the entire lineart.
Step 3. I then take the darkest colour and apply it to the top of the horse using the Airbrush tool. You can vary how much of your horse is Sooty, but I recommend using photos of real horses for reference. I then go to 'Filters' - 'Blur' and click on 'Guassian Blur'. You can choose the levels you want, but i normally set both values to 10.0.
Step 4. If you want to, you can also add dapples to your horse's coat. To do this, take the lighter colour again and use the Galaxy brush and the Airbrush, apply it to the darker areas of the horse's torso and/or neck. This is the Galaxy brush I use for dapples:
http://www.littlewebhut.com/gimp/brushes/images/Galaxy_AP.gif
Step 5. Fill in the rest of your horse's details, merge the layers together and either make the background white or apply your finished artwork to a background.
Blossom and render (c) me