The ballast discriminator works by examining the light frequency coming off the target lights. Here in the USA, if you point the discriminator at an incandescent light bulb which is being powered by a 60 Hz source, the circuit should indicate with the orange led. The discriminator photo detector is detecting a 120 Hz signal (since light is not polarized, the freq is doubled). This portion of the circuit passes the signal to a low pass filter with a cut off frequency of around 360 Hz. So the orange led will indicate so long as the light source frequency is 180 Hz or less.
For electronic ballasts, the supply to drive the lamps is usually converted to a frequency of 15 kHz or greater. The discriminator photo detector should detect a signal of 30 kHz or greater, which then gets passed to a high pass filter with a corner frequency just below 30 kHz. This will indicate the green led.