NegativeScreen is a tiny free utility - a 91KB download - which can reduce eye strain by inverting your screen colours on demand.
Launch the program and you're able to toggle the "negative" effect by pressing the Win+Alt+N hotkey, or right-clicking NegativeScreen's system tray icon and choosing "Toggle Inversion". Either way, your display should update right away.
This can make colours look a little weird, but NegativeScreen offers various other inversion modes which might be able to help. Some try to invert black/ white shades but keep colours more or less the same, while others don't actually invert colours at all (there are red or greyscale modes).
If the effect gets in the way at any point, toggling it off will return you to normal immediately.
A detailed configuration file allows you to customise the program hotkeys, update or extend the various inversion modes, and apply a few other tweaks of your own.