My question though is, when you go to reroll a horse it says all genes will remain the same. So how come my buttermilk dunalino tobiano splash mare became a Dunskin Tobiano Splash all of a sudden? That seems like a gene change to me.

The genes are the same - the variations are different.

"Buttermilk" buckskin is a buckskin based on light bay. Light bay, blood bay, mahogany bay are all just bay with differing shades of red. Pale palomino is just a light palomino. Liver chestnut is a dark chestnut. Etc, etc. All those variations have the same genes, just varies from horse to horse how it's expressed. These colors are randomly given to horses of the "base" genetic color. So people usually reroll horses to GET them, not reroll horses WITH them. "Reroll" is to roll the virtual dice that gives an alternate shade of color or a particular marking again to try for a particular result. You can try to reroll the buttermilk shade if you want. Good luck.