How to get the golden guns?

I've been playing the heck out of this game for the past few days, and I noticed some players had some nice shiny golden guns.

What conditions do I need to fulfill in order to unlock these weapons? (Are the they tied to the story mode, Amiibo, etc.)?

You can get the Gold Dynamo Roller by defeating the final boss and taking his scroll blueprints. You need to reach level 20 to be able to buy the weapon, though.

To get the Aerospray RG (the golden rapid fire gun) you have to beat the Octowhirl and reach level 13 to be able to buy it.

The amiibos do not unlock golden weapons. They unlock replicas of the weapons used in single player (Hero Splat Roller Replica etc.)