
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.

bright / garish
Having a very intense, bright, and often obtrusive color.
Sie trug ein grell pinkes T-Shirt.
She was wearing a bright pink T-shirt.

shrill
Very loud, high-pitched, and piercing, so that it is unpleasant to the ears.
Das Baby stieß einen grellen Schrei aus.
The baby let out a shrill scream.
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.
bright / light
Hell and grell can both describe strong light. Hell neutrally states that a lot of light is present. Grell is appropriate when the light is so intense that it dazzles or feels unpleasant.

Die neue Lampe ist so grell, dass sie mich beim Arbeiten blendet.
The new lamp is so glaring that it dazzles me while I work.
Die neue Lampe ist hell und beleuchtet den ganzen Schreibtisch.
The new lamp is bright and illuminates the entire desk.