LightScreen

LightScreen is a free browser tool that fills a monitor, laptop, or phone with any color at adjustable brightness. Use it as a screen flashlight, reading light, ambient light, or red light for stargazing. Extra modes include breathing, RGB cycle, traffic-light blink, a ring light, and Zoom lighting. A screen test page shows solid colors for dead pixels and backlight bleed. Nothing is installed and no account is required.