• The name of the signal is read from the TOP POSITION DOWN
• A Green or Yellow color in the top position gives the basic indication - CLEAR or APPROACH.
• When RED is used with GREEN or YELLOW, RED is not considered because it contributes nothing to the name of the signal. It remains lighted to let you know it is in working order and to show the relationship of the other lights whether in the top, middle or bottom positions.
• If it's not all red, then it's not red at all
• A Green or Yellow color in the middle position indicates Medium Speed if the top position is green
• A green under red means clear medium speed
• In position light signalling, yellow under red means slow approach - expect next signal at stop. Slow speed until clear of all interlocking then medium speed applies.
• In position light signalling top red with yellow position lights sloping away from the track means restricting.
• A Flashing Green or Yellow color in the middle position indicates Limited Speed
• A Green in the bottom position indicates Slow Speed
• Y/R/G = approach slow
• Y/Y/R = approach slow (this alternative does not follow logic, and in some other jurisdictions means advanced approach)
• When RED is in the top position the signal name begins with a speed - EXCEPT
  • R/Y/G medium approach medium
  • R/R/FY slow approach
  • R/R/Y restricting
• Flashing signals generally upgrade a signal, except maybe top flashing green which means proceed at CSS speed, or 60mph if not CSS.
• Flashing top yellow means advanced approach - expect approach at next signal

https://rail.pgengler.net/signals/fixed_norac.html

http://www.blet375.com/csx-Signals-REDI-Center.pdf