On US mainlines, signals at interlockings (CTC) are always red when the block has not been cleared by the dispatcher. Once the block is cleared for a train, the signal will show the correct aspect.
"Automatic" signals, which are not CTC (dispatcher) controlled are normally dark when no train is approaching, showing an aspect only when there is a train in the approach block.
I have found that Trainz signals do not accurately depict US mainline prototype. A signal will not show the correct aspect as soon as a train enters the block (between the previous signal and the next), but instead begin to display when the train is at some specified distance from the signal.
I have also found that signals will display varying aspects and messages (when moused over) when the block ahead is unsignaled or closed (switch thrown against, etc). It depends on how the signal is scripted.
Not being really familiar with Trainz scripting, I have no info on this.
I am glad that you figured out your issue on your own. You always learn more by solving a problem yourself than letting someone else do it.
FW