Railroad signals are red, if lit, for safety purposes, until an oncoming train has another signal (yellow, green or combinations, as applicable), as determined by the dispatcher. The reason being that there is no reason for them to be any other color than red, unless an oncoming train has the right to pass by that signal. Red signals are a safety issue.
Train signals are much like highway traffic signals. A Red traffic signal keeps you from crashing into a car coming from a different direction. Only when you gain the right to proceed does the signal light turn from red to green. Same basic setup with railroad signal lights.
Lately, some red signals are being replaced with "approach-lit" signal lights, which are dark (no power) until activated by an oncoming train. Then, the signal lights display the colors as determined by the dispatcher. Once the train has passed by, the signals then go back to dark, as an energy and bulb-saving measure
Hope that this explanation helps.
Take care and best wishes!