Classical problem of aesthetics
In many cases, the GTFS data set has no specific color for the transit lines (routes) or it has too few of them. Sometimes, there is one color. Like Mr. Ford said, “you can have any color so long as it is black”. And as you know this is just a poor excuse for someone who is pessimistic. In that case, one can choose to use a custom made palette of colors generated for that special occasion.
A much debated issue
Internet is full of tutorials about how to generate colors that look good. Just search with your favorite engine. You will most certainly find the endless discussions about the color models, the cube vs the cylinder vs the sphere (RGB, HSL). Then, many will raise the problem of color spatial distribution. A good palette of colors has individual elements that are evenly separated according to whichever color model is used. Wikipedia has an interesting page.
Ready made solutions
Next step is to find color generators. Today, plenty of people are eager to share code with the world, especially when it comes to web page color management. Stackoverflow is a gold mine.There are tons of answers. Then, there are the ready to use tools, sites that take a visual approach, like Color Space or Paletton. The problem lies in the numbers. It is relatively easy to produce up to half a dozen colors that look good.
Real needs
A transit agency has much more bus lines. Many have dozens of different routes, some have hundreds of them. So, just six colors is a poor choice. One needs more than that. 12 is a nice choice, 20 is even better. All depends on the visual identity of that agency. Ideally, the number should depend on how many bus lines meed at most at the busiest nodes of the network. The node can be either a terminal hub or a large square.
Any agency has designers. They manage the identity of that company and are responsible among other things of the color management. Still, I have seen many GTFS sets which lack systematically route colors. Enters XSTS.
Will continue …
The Spline 