It looks like you are using an HSV pallet on the single layer bitmap mask called the gunsight. Choosing a color then pulling the V vector as close to black while still retaining the color befor the value is translated by the gunsight mask area as clear. Somewhere between 2 and 10 depending on the color.
This gives a polarisation illusion something like the blue grey sheen on the F4U-1x families reflector glass plates. But, there is some novelty to finding the right shade of the primary color wheel that when you pull V to almost 1 the translation in the game is to cancel some of the blur at the edges of the con's image giving a crisper outline and contrasted body. Depending on the video card, monitor and customers eyes. Results will vary. Reds through yellow are good for ground. Blues through green for air to air. Cyan gives good general results. But, clear is clear and you can't miss from 100yds whatever the color or alpha value.
Simliar rational for amber polorised lenses on sniper scopes for low light conditions.
