What color are the vision blocks, etc. on Abrams (M1A2's)? Flyhawk indicates they should all be red. I Googled images and some appear red, some blue, and some even green! One tank in particular had each of the driver's blocks in different colors! Do they appear different color from different angle light reflection? Is there just one color that they all are?

Exactly, the actual glasses are coated with a protection against laser emission which is reddish.
A good way of depicting this is to paint the plastic black and add a number of clear red layers till you're happy.
If you use clear plastic for the glasses, paint the innner face clear red first and then you can paint the black.

Olivier

These are another option that work great
https://www.ebay.com/itm/AFV-Club-1-35-AC35016-Sticker-Anti-Reflection-Coating-Lens-for-M1A1-M1A2-Abrams/142051314286?epid=1148907658&hash=item2112eb0a6e:g:mJwAAOSw3mpXMVhs

Thanks. I've already painted the inside surfaces of the clear parts red, so all I have to do is paint a final coat of black.

This is using blue sequins that are cut down into shape. using orange or red works good also. All will show different shade of the color depending on the angle and reflective light highly like the original.

I also use sequins, but use different colors, which I picked up on the cheap at the local crafts store, to simulate the color changes based on viewing angle.

I haven't used any products from the line, so I have no idea how suitable these would be, but there is a 'Color Shift' line in the FolkArt paints, which "project a metallic luster that shifts in changing light". There is a product page on the manufacturer's website; at the bottom of the page is a link to a video showing off the product. Unfortunately, the video is excessively short, but about five seconds in you see a row of vases painted with the different colors, and can see the color shift.

Also worth noting that when viewing directly into these vision blocks from outside the vehicles, the 'glass' doesn't show any of the anti-laser qualities as when viewed from an angle

Another advantage to sequins is the reflecting quality as you can see with the drill bit. If you were to take a pic with flash directly in line you would get a effect like a mirror also.