One tone. That is the whole difference between the two commonest answers in ear training: the note sitting between the bottom of the chord and its fifth moves down by a single semitone, and the brighter triad becomes the darker one. Everything people say about the pair — bright against dark, open against inward — is a description of what that one semitone does, not an extra fact about it.
What to listen for
Do not listen to the whole chord. Listen to the middle. In the brighter triad the lower gap is the wider of the two — four semitones under three; in the darker one they swap, three under four. The outer distance is identical in both, which is exactly why judging the chord "as a whole" is unreliable and why beginners guess by mood and then get contradicted by the answer key.
The nearest mistakes
When one of the two triads is answered wrongly, the answer is usually the other one, a suspended chord, or a bare fifth — every one of them the same chord with a single tone moved or removed. That is not a coincidence, and it is the reason the quality pages compute a near-miss set from the interval stacks rather than listing "commonly confused" chords by reputation. Drill a quality against the chords one tone away from it and the errors have somewhere to go.
Mood is a consequence
It is fine to use "bright" and "dark" as handles; they are quick, and quick matters when a round is playing. Just do not let them become the method. Mood judgements collapse the moment a chord appears in an unfamiliar register, at an unusual tempo, or with a timbre that has few harmonics — which is why the drills here vary all three. The stack survives; the mood does not always.
The timbre trap
Rich timbres do half the work for you. A sound with many strong harmonics reinforces the tones of the chord and makes the middle interval easy to pick out; a nearly pure tone gives you almost nothing beyond the pitches themselves. That is why the hardest setting in the quality drill is not a rarer chord but a plainer sound — the answer has not changed, only the amount of help.
If a level suddenly feels much harder after a timbre change, that is the exercise working. Stay on it for a few sessions before deciding the level is above you; the accuracy usually recovers, and what recovers with it is a judgement that no longer depends on the instrument.
Where to drill it
The pair is the foundation tier of the quality drill, and the third that separates them is the third you can practise on its own in the interval drill. Doing both in the same week is the fastest version of this.