I know about the green card but I heard this term "yellow card" from relatives that s/he gets a yellow card. They are saying that it means s/he has no problems for eight years. Is it true? Is there any term like yellow card or similar in the process of getting a green card?
Whoever is telling you this is conning you. There is no such thing in the US. If it had been the UK, it could mean a yellow registration certificate for Croatians who are going to study.
But in the US? No.