This is the definitive site for Harrier info:
http://harrier.hyperlinx.cz/
On there is all the info you need, including a list of all the different paint schemes, by tail number, carried during the Falklands war. As well as a complete bibliography of available references.
Essentailly though, most wore an all over scheme of Gloss Extra Dark Sea grey (humbrol 123 or Gunze Sangyo H333). This was the colour worn on the top surface pre war, and all that happened on the way south was that the white on the underside was over painted. Most of the markings were also painted over, and the white in the roundels was filled in with dark blue.
Only eight aircraft, ( those that arrived aboard the Atlantic Convayor on the 18th of May) were painted in the Barley grey (GS H334) lower and Meduim Sea Grey (GS H335)upper scheme , this is a much lighter grey colour. These had black codes and pale blue/pale red roundels. After the war, all aircraft were repainted in Dark Sea Grey.
As for internal colours, Medium Sea grey for the cockpit and seat, front face of Seat Headbox should be black, top of the head box, seat cushion and backpad olive green, straps sandy brown.
Check out the above website for any other questions, and check out
http://www.ipmsstockholm.org/helpdesk.asp
For colour matching to what ever brands of paint you can get.
Hope this helps