I was reading this article:
In Eyes, a Clock Calibrated by Wavelengths of Light
It points out that blue light is what suppresses melatonin production and keeps one alert. And to suppress melatonin less, the light color should be like yellow before going to sleep.
It also pointed out that the current LED backlit screens produce twice as much blue spectrum light as the older CCFL backlit screens. The best colour for your Desktop background would be pure black, thereby blocking as much light emitted by the LED backlighting as possible, (assuming you have an LED backlit screen).
I have most program windows as dark as possible and still be comfortable with the visuals, (purely because I hate glare), and the Desktop background picture changes every 15 minutes between a large number of photo and rendered images, in theory evening out any exposure to a single wavelength for any prolonged amount of time.