
Thursday will see Arsenal wear a kit combination which hasn’t been seen yet this season as they switch to yellow shorts for the trip to Southampton.

Earlier in the campaign, the default change kit, with navy shorts, was worn away to the black shorts of Newcastle United and Sheffield United, but there has yet to be a shorts-clash since the Premier League resumed – Arsenal uncharacteristically wore their home kit at Brighton & Hove Albion on Saturday, despite changing there in recent years.
The Premier League handbook doesn’t lay out hard and fast rules on shorts, but does reserve the right for the league to adjudicate:

It’s the first time since the 2014-15 season that Arsenal will don yellow shorts with yellow shirts and the first different example of such an ensemble.
Prior to 1991, Arsenal wore the home white shorts with their away shirts when such a clash arose. The Football League had brought in a shorts-clash rule in 1975 and , oddly, the first such instance for Arsenal was at Liverpool in 1976-77. Generally speaking in the first adidas stint, it happened at Sheffield United, Southampton and Sunderland.

Ian Wright’s league debut in September 1991, a 4-0 win at Southampton in which he scored a hat-trick, was the first time yellow shorts were worn, paired with the ‘bruised banana’ away shirt.
That mashup also appeared at Bramall Lane that season but, when the Premier League began in 1992-93, the shorts-clash rule wasn’t applied – so as to encourage more wearing of home kits. It meant that all-yellow wouldn’t be seen again until 2010.
In 1999-2000, the white home shorts and change white socks had been used with the yellow change shirt against Barcelona and, while yellow alternative shorts were available in 2000-01, the were only ever worn with the navy third shirt against Mallorca at the start of 2001-02. Similarly, the 2008-09 away kit had alternative yellow shorts and navy socks but neither set was called upon.
For 2010-11, yellow was paired with redcurrant shorts for the change kit but the yellow back-up shorts got more than a few outings, including in 2011-12 for its only usage as a third kit, against AC Milan, albeit with a different crest to mark the club’s 125th anniversary.

Nike’s last change kit was in the classic yellow and royal blue colourway but the primary shorts were always worn, even at Crystal Palace, when plain white change socks were used.
A year later, Puma provided a yellow and navy second kit but, this time, the shorts – and hooped socks – were changed for the trip to Selhurst Park – presumably the navy sleeves were a factor in looking to maximise differentiation between the teams.
I would have loved to see the 08/09 away kit with yellow shirts – I have Arshavin on the back thanks to his memorable game at Anfield