Horizontal yellow and pink lines on desktop macbook pro

Hey everyone,

All of a sudden horizontal yellow and pink lines appeared on the desktop of my mac. The lines are exclusive to the desktop wallpaper and don't run over any other program.

Pic: http://i.imgur.com/Ysz9f8o.png

For reference: Macbook Pro (13-inch, mid 2012), 2.9 GHz i7, Intel HD Graphics 4000, 8GB Memory, 256GB SSD, OS X Yosemite 10.10.4.

Does anyone know what might be the problem?

Thanks.