Is there a master list of all the different color genetics that are possible with each breed? Also, is there a guide or forum post that has actual picture examples of all the different possible images for all horse color/breed possibilities?

We are going to have this. It will be in the new wiki. It will have example of colors, but not every shade/pattern/marking available. That would be a bit much as there's thousands.

If you look under Breeding Discussions and click the Genetics star underneath, you'll find several attempts at color/pattern guides. You can get an idea of just how humongous a really comprehensive one would be and you can see that several players put in tons of work to just burn out one day trying to keep up with changes.

You can google up any color/pattern you're curious about and choose Images to see real horses with those colors and patterns to get an idea of what your pixel horses might look like.

Also, under the Game Guide you'll find this:

Genes Appearing In Breeds

• Currently Registered Irish Draught(RID) horses do not have Frame.
• The only captured horses that can come in Appaloosa are Sport Horse, Sport Pony, Irish Sport Horse, and Knabstrupper.
• Knabstruppers always have Appaloosa coats and aren't dominant for grey or paint genes.
• Thoroughbreds used to be able to be captured with Tobiano and Dun. This has changed, and you can no longer capture TBs with Tobiano or Dun
• Anglo Arabians cannot be captured with Tobiano or Dun
• Only Sport Ponies can be captured with Mushroom, and the gene can be bred into ISH and SH.