Exposure - you are doing it right to get the maximum colour in the bursts. I had to pitch to get some detail in the people and the scenes, which means a lot of the colour is lost from the fireworks. f/11 is a sensible aperture for typical pro fireworks. We ended up in Alicante by mistake, and did not realise what was happening there. Every night for a week they had carnival parade processions, street fireworks and a big firework competition at midnight (slightly too late for the best sky effects). On June 24th, hundreds of huge sculptures (brilliant artwork, made of wood, polystyrene, papier-mache and other flammable materials) are burned in the early hours of the morning. The areas are closed off - only members of the Fogueres committees and teams, and invited guests get to see this properly. They are well prepared and have to be, because despite their being dressed in ballgowns, suits etc the 'bomberos' turn the fire hoses on the crowd to keep them cool while the huge statues burn. Some people just take off most of their clothes; digital cameras were getting soaked, even the TV crews were not immune.
Here is the main Fogueres statue:

- Fogueres de San Joan - the spirit of Alacant as a sea-nymph
Here is the only view we could see of it burning at midnight on the Eve of St John:

- The town hall with the burning figure reflected in an office block
And here is what remained an hour later:

- Burned and damped remains
We don't tend to return to places - ever - but spending this fiesta week in Alicante with a foreknowledge of the routine, where to be, what hotel room to book (there are several overlooking this square), and what gear to take (Olympus waterproof compact digital methinks) would be something worth doing again.
Also, we were up until 3am every night as the town never really slept. People were drinking, eating, playing music and letting off firecrackers (very loud, Chinese style, thrown in the streets). Spain also scored an important semifinal soccer victory the night before and they just added to the celebrations by driving through the city, every street packed, sounding their horns constantly. There was no point in trying to sleep
David