Click "Show Options", then:

- Check "Show chords on piano?"

- Check "Play chord sounds"

- Uncheck "Play feedback sounds"

For the "Chords", start with just Red and Blue. Then add Yellow.

I don't have perfect pitch, and I could not distinguish between Red and Blue, when Yellow was in the mix and the feedback sounds interrupted the trials.

Black and Green were much easier for me to differentiate, but Red / Blue is really difficult.