The referendum was not just illegal, it was clearly rigged. The local parliament lack the power the vote for independence, that's just a farce.
From google:
> a country or area under the full or partial political control of another country, typically a distant one, and occupied by settlers from that country.
From wikipedia:
> In politics and history, a colony is a territory under the immediate political control of a state, distinct from the home territory of the sovereign.
If Spain asserts itself as the ruler of Catalonia, without the will of the Catalans, it appears to me that the word is very apt.