The mod has an option to use the default colorful trails for "unpainted" trains (which, I believe, is the rainbow trail). I'm also using this with Automatic Train Painter, and I was hoping setting that mod to set empty trains back to the "default" red color (EA1100, according to that mod's settings tooltips) would invoke the "unpainted" behavior here, but after fiddling with it, noticed that this mod specifies that trains are only "unpainted" when first placed until changed by player or mod, and by implication, cannot be restored to "unpainted" status once that color has changed except by mining the train engine and placing it back down again.

So the request would be a new option field in the Map Settings section wherein a color code could optionally be specified, and trains that are not considered "unpainted" by the game, but are painted that specific color, would be treated as if they are unpainted for the purposes of the trails generated. This essentially mimics the same text entry field in the Automatic Train Painter mod where trains can be reset ("unpainted", though not really, apparently) to a specific color when they are empty of cargo.

Edit: Looks like I can set the "unpainted" to empty to make it use the default rainbow trails, which works. Would still be nice to be able to specify a custom color as "unpainted", rather than the default red.

I'm glad you found that the "unpainted" mod option can be empty, I specifically asked for that from the author and was so glad they obliged :)
Do you usually also have the Automatic Train Painter mod enabled? Train Trails already has so many mod options, I'm cautious to add more. But I'm thinking that I could also consider trains "unpainted" if Automatic Train Painter is enabled and the train has the color of that mod setting.

I almost always use it, yes. I added this one too after seeing in a stream, and was sad I wasn't getting the rainbows (because I had Automatic Train Painter set to a sorta neutral grey for "unpainted"). If it can directly read the setting from Automatic Train Painter, that'd probably handle it quite nicely, assuming some rival to that mod doesn't come about.

Great! I'll work on that for the next release.

<3

The update was released last night, thanks for the idea! I really like it. Any trains with the same color as the unpainted locomotive color mod setting will be considered unpainted and use the default color setting :)

<3! Much appreciated!