Electrical – LED color changing according to battery charge status

My question is does anybody know the forward drop on these protection diodes?

Read the datasheet carefully. The absolute maximum ist usually stated as VCC+X - the X is the forward drop of the diode.

[...] soltuion to this problem.

Switch the LED supply off at the same time, using 2 FETs. Alternatively you could use a N-channel FET to switch each LED.

Your root problem is that either you are using the wrong transformer or your mains voltage is too high. Replacing your transformer with a lower-output unit is your best bet.

Assuming that you really are using a 30 VCT transformer, rather than the 36 VCT unit which I suspect you have, replace it with a 24 VCT unit.

None of the solutions which you are considering will work terribly well. A zener can do the job, but you need a high-power device in order to handle the surge currents which occur at the mains voltage peaks. If you use 1 ohm resistor and a 20 volt, 10 watt zener you should be OK, but those things are expensive.

Something like an LM317/LM337 will not work well, since at full amplifier output you're running beyond their current capability, and the reliability of the circuit will be very poor.

Putting a resistive load on the supply to drop the voltage is actually the worst of your options. The problem is that it won't do any good if you don't have a load connected to the amplifier. Even during test you would need to maintain a load, or risk killing your op amp.

Using diodes to drop the voltage is just as iffy as using resistors, and for the same reason.