Without painting it on the background.
I got a blue flash from a decoder once. 
Jon
I saw a video on RFD-TV of an O scale layout somewhere that had a thunderstorm scene with lightning flashes. I think the narration said it was done by covering the back of a piece of plexiglass with black paper that had the lightning bolt shapes cut out, and flashing lights behind it.
I have a Circutron board that simulates an arch welder. I imagine you could use that and cut thin lines in a backdrop. You could get fancier and put a series of cuts in the backdrop and have the arc welder board move back and forth along the cuts. That way lightning would appear in different places.
a lot of modelers don’t really model a "bolt " of lightening per say, but try to simulate it with sound and lights …a strobe light facing the backdrop behind a mountain scene can do the trick…a sound system with a clap of thunder and a strobe light going off at the same time works well to simulate lightening…this works really well if you have a night-time (blue light ) appearance to your layout…chuck
There was an episode of Tracks Ahead that showed a layout that had thunder, rain and lightning flashes.
You could try doing something with static electricity. Perhaps you could have a motor rub to staticly sensitive materials together. Just a thought.
That could be a really bad idea with all the electronic equipment around model railroads these days.
On our old club layout we had a “thunderstorm” made from a piece of plexiglass cut to a “cloud” shape and cotton & poly fill glued to the front and painted with an airbru***o give the cloud that dark grey/blue color common to the storms here in the south. Behind the plexiglass was a very slow turning cam that would activate four microswitches as it rotated. Each microswitch was attached to a flash unit removed from old disposable cameras and placed in a various positions within the cloud. The microswitch would activate the charging circuit for each flash and when they built up a proper charge they would flash. Each one was in a different part of the “cloud” and the effect was awesome. We even had an endless cassette of a storm that would provide the thunder and rain effects. We tried illuminating a fiberoptic rod with another flash unit for a “bolt” effect but never got it to look right.
i have a back issue of model railroader not sure how far back it is where the author did a thunderstorm scene for his ho scale layout i think it was, check the index of magazines at the top of the page
hope this helps
tom