Here's the explanation about Braga though from Arsenal.com
Why Braga fans are known as 'Arsenalistas'
Way back in 1921, Braga changed their original colours of two large green and white vertical stripes to red shirts and white shorts.
Why? Because their then-coach Jozef Szabo was so enamoured by Arsenal during a trip to Highbury that he decided to adopt the London club's colours.
Such was the Gunners' influence on Szabo, Braga even renamed their youth team Arsenal do Braga. And ever since their club changed its colours, the Braga fans have been known as Arsenalistas.
