That was a conscious choice made by Gulf Air. There's a good story behind it.
Gulf Air was founded from the remains of BOAC's operations in the former British colonies in the Gulf(namely Bahrain, Qatar, UAE, and Oman). These four states each purchased 25% of the company to create a flag carrier for the Gulf region. This meant a new brand identity was needed to represent each of the Gulf nations. They ended up settling on the lovely Golden Falcon scheme, characterized by its use of gold titling and three main colors; Violet, Green, and Red.
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Close up shot of the tail, where all the colors of the Gulf Air scheme of the era is visible.
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The original violet color was meant to represent the State of Qatar, while the Red and Green were representative of the UAE, Oman, and Bahrain. This scheme lasted until the early 2000's, when Bahrain took full control over Gulf Air as its flagship carrier while the other Gulf States withdrew and created their own Flag Carriers(Oman Air, Qatar Airways, Emirates, and Etihad).
Fast forward to 2017, the Gulf States have this massive falling out with Qatar, accusing Qatar of working against the interests of the other Gulf States. Qatar was shunned by the others, airspace was restricted, and the Saudi's threatened to turn Qatar into an island by digging up their land border with them. As Gulf Air was now the flag carrier of Bahrian, it had to follow its government in shunning Qatar. So when Gulf Air's 70th anniversary came along in 2019, they wanted to paint an aircraft in its iconic retro scheme. But political considerations meant they couldn't. Painting the violet on the aircraft would be a tacit recognition of Qatar, so it was replaced with gold, which made since since "Golden Falcon".
Long story short, the reason why the gold was used and not the historically accurate violet was because Bahrain didn't want to acknowledge the Qatari role in Gulf Air's history due to a silly political dispute.