I know that it was under dominion of Mexico in the period predating the bear flag revolt, but was Mexico still a colony/province of Spain at the time?
Mexico started its war of independence in 1810. That more or less wrapped up in 1821, with the signing of the Treaty of Córdoba. 1821 is still celebrated in Mexico as the year of independence.
The Spanish Cortes however never approved the treaty. Spain wouldn't officially recognize an independent Mexico until 1836. There were a series of minor actions between 1821 and 1829. But Mexico was de facto independent by 1821. The US recognized Mexico in 1822 when president Monroe received the first Mexican representative to Washington. In 1825 the US sent their own minister to Mexico City.
The border between the US and Mexico was established with the Adams-Onís treaty of 1819. This was confirmed by the 1828 Treaty of Limits between Mexico and the US.
So by the time the Bear Flag revolt broke out, Alta California was recognized as Mexican by Spain, Mexico, and the US.
Thank you so much.
It does me no end of happiness to mess with the anti-imigration crowd here in CA by reminding them that we live in the old north half of Mexico.