I'm not strictly opposed to adding this, but I can't find a lot of information online and to me, it looks like this might just be a sporthorse breeding program based in Croatia vs an actual breed with breed standards and a studbook. For example, one of the horses you linked for color evidence has what looks like a Bavarian Warmblood or Holsteiner brand on his hip. If you can provide any information in English please share it with me so I can have a look- I haven't found many hits for Croatian Warmbloods. You also need a lot more color evidence for the modifiers you've listed, and proof that the horses are actually Croatian Warmbloods. There are a lot of registries that claim they accept "all" colors, but unless there's evidence of it actually occurring in the breed we don't accept it through realistic registration. It would be very difficult for a small, fairly new breed descended from Holsteiners to end up with Champagne (which is an American dilute) and Pearl (which is Iberian in origin). False Dun (nd1 and nd2) need to be proven with genetic tests. At this point, I would only accept the base colors and grey along with white spotting for this breed, and I still would like to see a bit more registry, studbook, and outcross information for this breed before I add it.
You are also missing the 3 examples of the breed created in your game.