It will be necessary to display a sticker in your vehicle windscreen showing what category it is in for emissions. They can be obtained from Post Offices for 5€.
Failure to display the sticker incurs a 200€ fine if you have entered a city where they are required with populations over 50,000 such as Alicante, Valencia and Gandia. More cities with lower population levels will be added in future.
Check the DGT website https://sede.dgt.gob.es/es/vehiculos/distintivo-ambiental to see what sticker your vehicle will have.