Thank you all
Dyes are not what I wish to do, since it will make the fabric more coarse..
The bleach direction was more of what I was looking for, since what i want is to get it brighter.
The problem is that so far, from my experience, Polyester never reacts to bleach.
I also tried it on a small part that I cut from the pants' pocket,
and indeed, no reaction at all..
So seems like these pants' color is not changeable..
Thank you for trying to help