Activate the Adjust Luminosity option to access the Luminosity slider, which lets you change the brightness of the material.
When the color is set to Custom Color, the Luminosity slider is disabled.
A value of 0% changes the material to pure black. A value of 100% uses the same material color as applied in the Materials settings. A value of 200% changes the material to pure white.
The Luminosity slider only works when the Color Source menu is set to Material Color.