leuchtend
bright
Describing a very intense, strong, and vivid color.
Sie trug ein T-Shirt in einem leuchtenden Gelb.
She wore a T-shirt in a bright yellow.

bright / shining
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bright
Describing a very intense, strong, and vivid color.
Sie trug ein T-Shirt in einem leuchtenden Gelb.
She wore a T-shirt in a bright yellow.

'Leuchtend' (bright/shining) is the present participle of the verb 'leuchten' (to shine, to glow). This form is used as an adjective and keeps the basic meaning of radiance, whether with colors, light, or in a figurative sense.
Die Glühwürmchen leuchteten in der Dunkelheit.
The fireflies glowed in the darkness.
To describe an intense color, 'leuchtend' is placed directly before a color adjective. Typical examples are 'leuchtend rot' (bright red), 'leuchtend grün' (bright green), or 'leuchtend blau' (bright blue).
Das Plakat war in leuchtenden Farben gedruckt, um Aufmerksamkeit zu erregen.
The poster was printed in bright colors to attract attention.
When 'leuchtend' refers to light, it often describes things that shine by themselves or strongly reflect light, such as 'leuchtende Sterne' (shining stars) or 'leuchtende Augen' (shining eyes) from joy.
Als sie das Geschenk sah, bekam sie leuchtende Augen.
When she saw the gift, her eyes lit up.
The figurative, exemplary meaning is used almost only in the fixed expression 'ein leuchtendes Beispiel' (a shining example).
Sein unermüdlicher Einsatz für andere ist ein leuchtendes Beispiel für Zivilcourage.
His tireless commitment to others is a shining example of civil courage.
dark
'dunkel' and 'leuchtend' are often opposites when it comes to light or colors. 'dunkel' describes little light or a color that is not light. 'leuchtend' describes something bright, radiant, or a very vivid color. With a room, a lamp, or a color, the difference is clear: 'dunkel' feels weak or close to black, while 'leuchtend' feels bright and eye-catching.

Vor der Reise hat Anna einen leuchtend roten Koffer gekauft.
Before the trip, Anna bought a bright red suitcase.
Im Flur ist es dunkel, weil die Lampe kaputt ist.
It is dark in the hallway because the lamp is broken.