Amix – the way the game works is the route has to work for all vehicles assigned to it. So if you assign an electric train to a non-electrified line, or one that is not completely electrified, the colour route line will disappear. In your example above, when you sent the electric train to the depot, it was removed from the line roster immediately and then the line colour reappeared because all vehicles now assigned could find a path to all the stations.
So simply, some part of your route has a gap in the electric overhead. To find it, use the electric upgrade tool, hover over track sections and they should show red. If it shows green, then it is not yet electrified.
If you see two pantograph towers close together, zoom in and there’s a good chance the short section between them is non-electrified. Also check cross-over’s too.