Samsung
Change Folder Color on Home Screen auf Samsung
By default, folders on the Samsung home screen use a neutral white or gray background. This makes it difficult to quickly identify a specific folder when you have several on one screen.
One UI lets you assign an individual color to each folder separately. This is a display setting tied to the folder itself, not a system-wide theme option, so changing one folder never affects the others.
To change it, tap and hold the folder briefly, or open it, then use this path: Tap on a Folder ➔ Tap on Color ➔ Select a Color.
You can pick one of the predefined colors or open the full color palette to set a custom shade. A slider also lets you adjust the transparency, so the folder background can blend more subtly into the wallpaper or stand out with a solid tone.
If folders still show no color options at all, the launcher may need a restart, or the device is running a very old One UI version where this feature was not yet available.
Side effect: a custom folder color is only cosmetic and does not change app icons, folder name, or the apps stored inside.