The chromatic partyhat is obtained from opening the mystery gift, purchased from Thessalia for 200,000 Zemomarks. It is a rainbow partyhat that is worn upside down on the head. Only access to New Varrock is required. It is not required to complete any of the sub-quests that take place there to obtain the hat.
It is the only item from New Varrock that can be removed from the alternate dimension. Right-clicking on the hat and selecting "Smuggle" will create a copy in the player's bank in Gielinor, while the original hat remains intact in New Varrock. When a player destroys their partyhat in Gielinor, it will still exist in New Varrock and can be smuggled out again.
There are multiple strategies for collecting the 200,000 Zemomarks required; however because most methods depend on a limited store stock, these methods should be rotated by priority.
When a player opens a mystery gift, a server-wide announcement broadcasts: " Player has bought a chromatic partyhat from Thessalia in New Varrock!"
Showing the chromatic partyhat to the Wise Old Man completes the achievement Seven Colours In Their Hat.