Consider the lighting at the player position when colouring the scope overlay.

For example.... :P

Since Flan hasn't given an example, I'll give one.

Okay, so here's an old G36C iron sight I made a long time ago. In daylight, it looks like this:

That looks fine, right? I think it does.
Now, in a cave (really just a box I made underground) with Moody lighting, it looks like this:

That doesn't look right.
I think that Flan is suggesting that zoom overlays (like iron sights) should be adjusted based on the surrounding light levels.