How to change the colour of gun sight?

Hello guys.

I was wondering how do i change colour of the sights HUD in the game?  

ok so i am using a sight that the HUD is green and i am wondering how do i change the colour of it so i can see it a lot easier.  In the game options i can change the HUD colour of almost everything so it makes thing easier to see apart from the HUD for the sights on the gun.  Why was this missed out?