From this motorbike guide:
Zona Blue: parking lines are painted in blue denoting either “pay and display” (pay for a ticket from the dispensing machine and display it clearly in the front windscreen) or display a blue parking disc set to the time at parking. Blue discs are available from banks, tourist offices, tobacconists and post offices. Always be aware of the maximum time allowed and confirm if payment is necessary as many blue spaces are free at lunchtime and in the evenings.
White Lines: mark the parking bays indicate that it is free parking.
Yellow Areas: indicate disabled parking (with a disabled permit/Blue Badge) or delivery zones
Another motorbike/scooter guide provides additional information:
Parking between two parked cars is fine, but only if you leave enough space for the cars to clear your bike or, if the cars are parked side-by-side, enough space for car drivers and passengers to enter and exit. In rural areas, one can park on the side of the road with impunity.