By default, OpenKneeboard uses a ‘full white’ tint; this is equivalent to full brightness. Other settings might be preferable for nighttime.
Tint is just a more general brightness control - a 50% gray tint is equivalent to 50% brightness - however brightness can be adjusted for any color.
Enable brightness/tint control in OpenKneeboard Settings -> Advanced
Tint color can be adjusted in Settings -> Advanced, and it can be disabled to restore ‘full white’; you can also increase or decrease brightness or toggle tint with: