The display brightness keys on my HP Folio do not function. 

The easy solution is to add the Brightness Applet to the Panel.  Right click the panel and select Add to Panel.  Select the Brightness Applet.  To use the applet, click on it to select it and use the keyboard arrow keys to adjust the brightness.

Adjusting Brightness, Contrast and Color Balance:

You can do this in Terminal by using the xbacklight and xgamma commands.  backlight has a range of 0 to 100% and gamma has a range of 0 to 1.0.  The gamma can be set as a group, or individually for red, green and blue.  Reducing the gamma increases the contrast.

xbacklight does not come installed.  In terminal, type xbacklight and follow the instructions for installing it.

To set 50% backlight, type

xbacklight -set 50

To set 50% gamma, type

xgamma -gamma 0.5

You can put these commands into a script file and give it  permission to execute as a program.