If the signal is just turning red, and at the same time, another signal from for example indigo->blue

I should write:

colorchanged, RED
stationchanged, INDIGO
colorchanged, BLUE

or directly

colorchanged, RED
stationchanged, BLUE

?

I think, the last variant is right.
Because when you disconnect from the station, distance from another station (where you connect) becomes EQUAL to BLUE_DISTANCE, and INDIGO is when this distance is LESS.

There are no such tests
If the signal is just turning red, then the distance to the closest signal change curve of the most powerful station is at least 10e-4*R, where R is the threshold distance of this station.