Why does the Xiaomi Watch S4 screen have slight color deviation when displaying at low brightness?

The slight color deviation observed on the Xiaomi Watch S4 screen at low brightness is due to the inherent properties of AMOLED materials. In AMOLED displays, the activation voltages for the red, green, and blue (RGB) colors differ, which can cause a slight color shift when the screen is set to low brightness. This phenomenon is more noticeable at lower brightness levels but disappears as the brightness is increased.
It is a normal characteristic of AMOLED technology and does not indicate a fault with the device. Therefore, you can continue using the watch without concern.