Robert,
Its actually a light grey caulking. With my Apaches, I simply hand painted the edge of the windows. Takes a very steady hand, but the irregularity of the brush stroke closely matches the irregularity of the caulk. That's not to say I globbed paint on though. I used a 3/0 brush and a VERY little amount of paint. I've got tons of photos of Apaches in Afghanistan and that's about the closest approximation I can do to match the photos. Point is, they're not the neat lines you'd find on an airplane. they're kinda rough.
If you go to my website, http://cobrahistorian.tripod.com and go to the photo album, the first photo should give you a good idea of what I'm talking about. That's "Devils Dance" from 1/229 AVN and will be profiled in my next book for Osprey.