Pants/trousers are plural. 'a pair of grey pants, some grey pants', or simply 'grey pants' would be correct.
Interesting. What's the difference between pants and trousers? I wasn't aware of one.
1) Can we have trousers as well as pants as the two are not the same and mean different things in North America and Europe/Asia? 2) Trousers or pants are always plural in English - So "A pair of grey pants" or "grey pants".