grell
glaring
Unpleasantly bright or intensely shining, so that it dazzles the eyes.
Die Scheinwerfer des entgegenkommenden Autos waren grell.
The headlights of the oncoming car were glaring.

glaring / shrill
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glaring
Unpleasantly bright or intensely shining, so that it dazzles the eyes.
Die Scheinwerfer des entgegenkommenden Autos waren grell.
The headlights of the oncoming car were glaring.

The word 'grell' almost always has a negative connotation. It describes something that is too intense and therefore unpleasant to the senses, whether light, color, or a sound.
Die grelle Reklame an der Fassade tut in den Augen weh.
The glaring advertisement on the facade hurts the eyes.
The noun that 'grell' describes reveals the exact meaning. In connection with light sources, it means 'blinding'; with colors, 'obtrusively bright'; and with sounds, 'piercingly loud'.
Das grelle Licht der OP-Lampe blendete ihn.
The glaring light of the operating lamp dazzled him.
When it is about light, 'grell' is often used for artificial light or very strong sunlight. Typical combinations are 'grelles Licht' (glaring light), 'greller Blitz' (bright flash), or 'greller Scheinwerfer' (glaring headlight/spotlight).
Die Mittagssonne war so grell, dass ich eine Sonnenbrille brauchte.
The midday sun was so glaring that I needed sunglasses.
With colors, 'grell' often describes neon or signal colors. Common combinations are 'grelles Pink' (bright pink), 'grelles Gelb' (bright yellow), or 'grelles Grün' (bright green).
Der Textmarker hinterlässt eine grell gelbe Linie auf dem Papier.
The highlighter leaves a bright yellow line on the paper.
In relation to sounds, 'grell' is used for high, loud, and unpleasant tones. Typical examples are 'greller Schrei' (shrill scream), 'greller Pfiff' (shrill whistle), or 'greller Alarmton' (shrill alarm tone).
Der Feueralarm ertönte in einem grellen, schrillen Ton.
The fire alarm sounded in a loud, shrill tone.
quiet
'leise' and 'grell' can form an opposite pair for sounds. 'leise' means that a sound is only weakly audible or at a low volume. 'grell' describes a very loud, high-pitched, and unpleasantly piercing sound. Important: 'grell' can also describe light or colors, but as the opposite of 'leise', it is about sound. If a sound is barely disturbing, it is 'leise'; if it is strong and unpleasant to the ears, it can be 'grell'.

Der grelle Alarmton weckt uns mitten in der Nacht.
The shrill alarm tone wakes us in the middle of the night.
Im Bus stellt ihr das Handy leise, damit niemand gestört wird.
On the bus, you set your phone to silent so that no one is disturbed.