Why is "milyen" appropriate here instead of "milyenek" (since "vonatokat" is plural)?
In Hungarian there is no "multiple plural", so "milyenek vonatok" is incorrect, while "milyen vonatok" is correct.
I believe it's because it is directly describing "vonatokat", which already is plural, so you use "milyen".
Milyen vonatokra vársz, pirosakra vagy feketékre? - should also be accepted.
My answer as well, it's accepted now.
What is wrong with my answer: Milyen vonatokra varsz, pirosokat vagy feketeket.
My first answer that I wrote out of indecisiveness... :D It's actually not right though, you should settle by one type of argument to use. If you decided to use várni +-ra/-re, be consistent and say pirosakra vagy feketékre.