Just an update from me. Converting the file to greyscale did not get rid of the colour patterns. I eventually settled on a version of the file darkened by 30% and with the 'visible' part of the image reduced by 50%. I still have the triangle, but it's softer and more transparent, and not unrealistic for the aircraft I fly (WW2 bombers). I even convinced myself that the patterns in the beam are consistent with the old style bulbs used in the landing lights on these old a/c 
