Agree with the battery tester idea. KISS principle.
I know just enough about electronics to be dangerous, but it would be far easier to modify the circuits to maybe make the existing red light flash when the battery voltage gets below a certain threshold. Even that is not a trivial undertaking. Making it light three different colors would be far more complex, including adding the other two LED's. That would almost certainly require a whole new circuit board for the preamp.
Example: I had a T5 Classic. Since it was rarely plugged in, the volume control pot (potentiometer) got very scratchy - it never moved. Electronic cleaning fluid (DeOxit) and exercise would fix the problem, but only temporarily. Instead of a new $10 pot to replace like any electric guitar, Taylor built the circuit board with integrated pots on the PCB. Oh, and we switched from one type of Molex connector to another since your guitar was built. You have to buy a whole new circuit board and wiring harness to use it. $114 for parts, plus labor by a Taylor certified tech, to do a repair that I could easily do in 15 minutes on any other guitar. Some of the things they do for manufacturing expediency make future repairs much more difficult. I sold the guitar with a full disclosure of the issue and the part number instead.
