Sometimes its hard to read someones signals...but you tend to develop that a bit as you get older I think.
I find sgal vary from what source you are taking them from...
In the motor world, Red means stop, green means go, and Yellow means warning, changes afoot...
*In England, French don't go Red->Yellow->Green...
In the train world:
red means stop.
yellow means the next signal is red, be prepared to stop
double yellow means warning, the block ahead is on a caution signal, be aware
Green means this block section is clear, you may pass
See - the basics of land-based signals...