A screen display apparatus and method of measuring an ambient brightness by adjusting color quality of an output screen in a mobile terminal according to the ambient brightness, so that the visibility of the screen can be ensured in various environments. The includes measuring an ambient brightness of the mobile terminal, and generating a visibility adjustment event when a measured ambient brightness value is equal to or greater than a preset specific external brightness value. The received RGB signal is converted into first tristimulus values, then to L, a, b color space valueslightness (L), chroma (C), and hue (H) values using a predetermined equation. The brightness of an entire region of the image is adjusted and output on the display screen of the mobile terminal, and the chroma of each pixel region is adjusted by using the L, C, and H values.