"My pants are not red."
Translation:Mis pantalones no son rojos.
That's interesting. When I was learning Irish, I ran into similar problems because the "English" was Hiberno English and I'm American. Sometimes I understood the Irish better than the "English." :-) The funny one was that American "eraser" was Hiberno English "rubber." The sentence to put it in context was, "He has a hard rubber." The book having been written by a nun, I was pretty sure that sentence didn't mean in Hiberno English the same thing it meant in American English! :-)
Possessive determiners work as articles. Therefore, 'las mis', referring to the same object, is redundant (and incorrect). Watch out! This was correct in old Spanish! xD (in some regional dialects this is how they speak).
And pantalones is masculine, if anything, 'los mis'.