einfarbig
solid-colored
Something that has only one single color and has no pattern or other colors.
Sie trug ein schlichtes, einfarbiges Kleid.
She wore a simple, solid-colored dress.

solid-colored
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solid-colored
Something that has only one single color and has no pattern or other colors.
Sie trug ein schlichtes, einfarbiges Kleid.
She wore a simple, solid-colored dress.

The adjective 'einfarbig' (solid-colored) is clearly made up of 'ein' (one) and 'farbig' (colored). So it literally describes something that has only one single color.
Ich suche ein einfarbiges T-Shirt ohne Aufdruck.
I'm looking for a solid-colored T-shirt without a print.
'Einfarbig' (solid-colored) can stand directly before a noun (attributive) or be used with verbs like 'sein' (to be) or 'wirken' (to appear/seem) (predicative). In both cases, it describes the quality of the noun.
Sie hat sich für einfarbige Vorhänge entschieden.
She chose solid-colored curtains.
This adjective is often used to describe clothing, fabrics, or surfaces. Typical combinations are 'ein einfarbiges Hemd' (a solid-colored shirt), 'eine einfarbige Wand' (a solid-colored wall), or 'ein einfarbiger Teppich' (a solid-colored carpet).
Für das offizielle Foto brauchen wir einen einfarbigen Hintergrund.
For the official photo, we need a solid-colored background.
simple / plain
'schlicht' and 'einfarbig' can seem similar when talking about clothing, fabrics, or objects, because both often look calm and not flashy. But the difference is important: 'einfarbig' only says that something has a single color and no pattern. 'schlicht' says something about the style: simple, without many details or decoration. A solid-colored dress can still have an eye-catching cut. A plain dress can be solid-colored, but it does not necessarily have to be. They can be confused because simple things often also have only one color.

Für den Kurs braucht ihr ein einfarbiges T-Shirt ohne Muster.
For the course, you need a solid-colored T-shirt without a pattern.
Leon kaufte für die Reise einen schlichten Rucksack ohne auffällige Logos.
Leon bought a plain backpack without noticeable logos for the trip.