When I power on my laptop (Fujitsu Siemens Amilo Pa) the screen turns grey and after a couple of seconds vertical lines of random color start to appear all over the screen. They change their color constantly. External monitor works just fine (through VGA connection).

I disassembled the laptop and checked the data cable that connects LCD and motherboard. I checked every single wire in the cable (rungup with my multimeter) and the cable is absolutely fine (I made a simple pinout schematics). Cable has a sticker: PTT50 LCD CABLE ONE, P/N: 22-12-023-70, CNJS: 2008 (some chinese? characters).

Photo of LCD with lines is in the attachment.

Will greatly appreciate any help!

The problem seems like hardware related. You need to take a hardware check up for the Laptop.