If you have a visual impairment such as ocular toxocariasis, you may find it more comfortable and less tiring to the eyes to work with a grayscale screen, since OS X usually displays a lot of colour in a normal use.
To make the display grayscale, go to System Preferences.
Click Universal Access
Under display check Use grayscale to convert the display to grayscale.
Before in colour.
After in grayscale.