You can easily creater 2 256 color .BMP files:
Gunsight.bmp - one 128x128 image containing the COLOUR of the gunsight
and
Gunsight_alpha.bmp - 128x128 image containing the transparency of the image.
A dot that is red (255, 0, 0) in Gunsight and white (255, 255, 255) in _alpha will show as a solid red dot
A dot that is blue (0, 0, 255) in gunsight and gray (127, 127, 127) in _alpha will show as a semi-transparent blue dot.
Save bpth images to the \sights subdirectory of your Aces High folder.
Easiest is to d/l some Gunsights and modify em. In AH select the "Gunsight.bmp", not "Gunsight_alpha.bmp", which will be automatically loaded.