My grandma puts a beet for the colour. And some horse-radish for… don’t know what. I think a corn cob was also used for something (though we don’t put it anymore). Basically, like with absolutely everything else, everyone does it slightly differently.
Btw, I’ve heard Serbs like eating and cooking with sauerkraut whole year round, including in the summer. Is that true? I personally consider cooked sour cabbage far superior over the fresh one, but it seems that the latter is generally the preferred one here.