all i remember is that the bad guys had purple space ships with which they attcked the peacful natives, and the space ships are invulnerable to the native's weapons, till one of them finds a ship with a sail made of gold (i think), that can concentrate the sun's rays and by that it can destroy the evil space ships.
and it's not "the lost cities of gold"...
anyone?

Are you really sure this is not The Mysterious Cities of Gold

This had got to be The Mysterious Cities of GOld, and the ship you are talking about is the sailing ship Solaris, the top of it's sail could be aimed and fired at other ships:



However, nothing spacy involved, and the Spanish Gallions aren't purple at all.

Spartakus and the Sun Beneath the Sea?