Plates with a yellow background are commercial vehicles - trucks, taxis etc.
Blue plates are PSV's (public service vehicles) - police, ambulance, fire etc
Red plates are diplomatic (embassy) vehicles
White plates with a blue square with a number on the right are vehicles owned by foreign registered companies, eg a blue square with a 12 is the Libyan branch of a German company.
Plain white plates are privately owned vehicles.
The Arabic writing on the plate translates as "Libyan Jamahiriya" or something similar...
This link is old, but shows most of them: