The aiming reticle is white, green, or red depending on what you are aiming at. Aiming down the sight causes the reticle to get stuck on whatever color it was when you started to aim down the sight. Switching weapons fixes the reicle until you ADS again.
Example: Look at a friendly such as Lilith. Reticle turns green. Aim down sight and then release it. Reticle is now stuck green until you switch weapons.
ADS while looking at nothing and the reticle gets stuck white etc.