"I am waiting for red buses and black cars."
Translation:Én piros buszokat és fekete autókat várok.
“a” is the definite article, similar to "the" in English. the original sentence does not have it, so it would be incorrect to use "a" here. besides, when one says "a buszokat", one has also to say "várom", the definite form, instead of "várok", which is the indefinite form.