This is indeed a very sueful info for me, as I'm still working in a new version of my interface mod for SH3
I had already noticed what you mentioned about the RGB and Alpha channel, as I had noticed it when modifying the red light mask. But what I didn't know is that adding the reticle as child to the existing "frame" in the menu-ini would make it turn dim green by night
So far I had solved it by adding a new image with the reticle in Alpha channel to the red mask layer, and as you suggested, tweaking the values to be bright green. But of course your new solution is a much cleaner method
EDIT: It's only sad that we can't get rid of the original marking lines, specially in the UZO!