Back when I had my astronomy-friendly outdoor lighting business, I used to sell yellow-LED flashlights that I bought from Robert D. Mantell in North Hollywood, California, under the trademark Lo-Glo™.
The Houston flood Memorial Day weekend 2015 wiped out the remaining inventory I had and, sadly, these wonderful flashlights are no longer available.
It is not rocket science. You need to start with a well-made flashlight, replace the regular bulb with a yellow LED and the appropriate current-limiting resistor, and voila!
Yellow may be better than red. See the article by Robert Dick, “Is Red Light Really Best?”, in the June 2016 issue of Sky & Telescope.
There’s a great business opportunity here. It wouldn’t take much to make a better astronomy flashlight than what Orion and others sell. Besides, I have found these yellow-LED flashlights to be most useful for moving around the house after bedtime (such as a bathroom trip) to avoid being exposed to any bright light at night which would affect your night vision and even your circadian rhythm.
If you know of any astronomy-friendly yellow LED flashlights or would like to manufacture some, please post a comment here or contact me directly.