"The big trains are not red, but black."
Translation:A nagy vonatok nem pirosak, hanem feketék.
In this structure, noun+adjective, the adjective also gets the plural ending.
If you are asking why "pirosak" and not "pirosok", that is is another story.
I thought the adjective of the plural noun gets the plural form too. Therefore big would be bigs?
Only if it is after the noun.
The big trains - A nagy vonatok.
The trains are big - A vonatok nagyok.