In description you're saying

Custom colors GUI. A click button in the upper left, opening a menu with an expandable list wherein you can select items and set their colors with RGB sliders like in the Train UI. A slow work in progress because I don't really know how to code GUIs yet...

But it's not really requried. You can use mod settings to allow user to set colors. For example I'd like to set different colors for Iron and Steel - now they are basically the same. The benefit is you don't have to write any GUI - user can just select it in settings just like Paint locomotives/Paint cargo and/or fluid wagons and other settings. Add bunch of settings like Copper Train/Copper Ore Train/Circuits Train/... with a textbox for rgb color description. It won't be as user friendly as custom GUI but it can be implemented in a day or so. Then it could be replaced with shiny new GUI with color picker and stuff in future but I think it worth making an MVP with what I'm proposing. People may just set train station color in game, remember it and then go to settings and write this number they got in game in settings so it will be automatically applied to the new trains.

Looks totally feasible and plausible to do. Hope you'll also see it this way