'pants' is a plural noun.
'trousers' is a plural noun.
(source: https://en.oxforddictionaries.com/definition/pants )
(source: https://en.oxforddictionaries.com/definition/trousers )
Your pants is black ==> incorrect verb conjugation (should be 'are' instead of 'is').
Regarding 'pants' versus 'pant':
'pant' (noun) = a short, quick breath.
'pant' (verb) = Breathe with short, quick breaths, typically from exertion or excitement.
(source: https://en.oxforddictionaries.com/definition/pant )
Your pant is black ==> Your quick breath is black.
Regarding 'Your black trousers':
In Indonesian, the noun comes first in a noun phrase, followed by modifiers.
Examples of noun phrase:
celana hitam = black trousers.
celana kamu = your trousers.
celana hitam kamu = your black trousers.