You write with kanji like that 何こかのボールは黄色です. It means " are some balls yellow? "

"何個（なんこ）~" - means "How many ~" if it is phrased as a question. However, if you add か to the interrogative phrase 何個 then it becomes "some ~" and the phrase 何個か is treated as an adjective that can only be connected to a noun with の. So, the full phrase is saying "Some balls are yellow." This is the impression I get at least if I am remembering my Japanese grammar correctly. The same can be done with any counting word. 何枚かの紙　　Some sheets of paper 何本かの鉛筆 Some pencils I hope this explanation is clear. If you need clarification, let me know.

I am not sure but I guess it means " The ball of Nankoka is yellow"