Essentially it’s exclusive to events that offer it or paid things in the shop like bundles and such. Otherwise the free to play is mostly the purple ones. You don’t need the gold specifically for those levels, it’s just showing the gold cause that’s what it needs to finish the level with the extra blue and green added. Otherwise you just need any amount of the other ones, you don’t exclusively need gold

that's event exclusive and not farmable, as for the event its whatever