dezent
subtle
Unobtrusive, restrained, and not intrusive in color, form, volume, or behavior.
Sie trug ein elegantes Kleid in einem dezenten Grauton.
She wore an elegant dress in a subtle shade of gray.

subtle / discreet
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subtle
Unobtrusive, restrained, and not intrusive in color, form, volume, or behavior.
Sie trug ein elegantes Kleid in einem dezenten Grauton.
She wore an elegant dress in a subtle shade of gray.

'Dezent' (subtle/discreet) means 'unauffällig' (unobtrusive) or 'zurückhaltend' (restrained). It is a false friend of the English word 'decent', which means 'anständig' (proper) or 'akzeptabel' (acceptable).
Für das Vorstellungsgespräch wählte er ein dezentes Hemd.
For the job interview, he chose a subtle shirt.
'Dezent' (subtly/discreetly) is very often used as an adverb to describe how an action is carried out: in an unobtrusive or tactful way.
Sie hat ihn dezent darauf hingewiesen, dass sein Reißverschluss offen war.
She discreetly pointed out to him that his fly was open.
In the meaning 'unauffällig' (unobtrusive), 'dezent' (subtle) often describes visual or acoustic impressions. Typical combinations are 'dezente Farben' (subtle colors), 'ein dezentes Muster' (a subtle pattern), 'dezenter Schmuck' (subtle jewelry), or 'dezente Musik' (subtle music).
Die Wände sind in einem dezenten Beige gestrichen.
The walls are painted in a subtle beige.
When it is about tact, 'dezent' (discreet) is often connected with words of communication. Common examples are 'ein dezenter Hinweis' (a discreet hint), 'eine dezente Frage' (a discreet question), or 'dezente Kritik' (discreet criticism).
Mit einer dezenten Geste bat der Kellner die lauten Gäste um Ruhe.
With a discreet gesture, the waiter asked the loud guests to be quiet.
simple / plain
'schlicht' and 'dezent' are close in meaning when it comes to style, colors, or appearance. Both words can mean that something is not conspicuous. However, 'schlicht' mainly emphasizes simple form and few details. 'dezent' emphasizes that something does not feel intrusive, for example a color, a perfume, music, or behavior. Learners may confuse the words because both often sound positive and calm. As a guide: if something is built or designed simply, 'schlicht' fits. If something is intentionally quiet, unobtrusive, or not too strong, 'dezent' fits.

Für das Gespräch trägt sie ein dezentes Parfüm.
She wears a subtle perfume for the conversation.
Im Wohnzimmer steht ein schlichter Holztisch ohne besondere Verzierungen.
In the living room there is a plain wooden table without any special decorations.