How do I calibrate my screen?
Calibrating your screen is very easy to do and should be a hard requirement if you're involved in computer design or artwork.
By using a device called colorimeter you accurately measure the different colors that your screen is able to display. By running System Settings ▸ Color you can easily create a profile, and the wizard will show you how to attach the colorimeter device and what settings to adjust.
Screens change all the time - the backlight in a TFT will half in brightness approximately every 18 months, and will get yellower as it gets older. This means you should recalibrate your screen when the [!] icon appears in the color control panel.
LED screens also change over time, but a much slower rate than TFTs.