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The Purpose of the Color settings panel is to provide tools for changing color settings for selected slide. There are 3 available settings (from top to bottom) - Brightness, Contrast and Gamma, for each setting there are 2 ways of change its value - slider and input. Slider and input are synchronized and user can adjust settings using both ways. 'Reset to default' button will reset settings to the following values - Brightness: 0, Contrast: 1, Gamma: 1.
Brightness refers to the absolute value of colors (tones) lightness/darkness. Increasing brightness of an image will light out all colors so the original light ones will become up to white. Reversely, decreasing brightness will darken all colors so the original shaded ones will become up to black. So, you can apply brightness to the slides accordingly.