I translated "... egy piros autó ..." by "... one red car" and got corrected. I thought since egy = one (the numeral), it could be used in this context. Is there a reason it's not possible here?
See my other reply to your similar question about the building above a/one river.
In Hungarian the context defines clearly whether it is a numeral or an indefintie article. In this case the "egy" is the latter, so you must use "a". (I am aware that sometimes English speakers use "one" as indefinite particle [like "you're one fine gentleman, sir"] but this is not a "proper" standard English.
The direct translation is confusing to me in terms of sentence structure/order of words.
Yes the order is strange if you're a native English speaker, but I think I'm getting used to it though!