It is an awfully dull colour for a team playing such a colourful brand of rugby.
The Grizzlies lead the competition standings with a perfect record of five bonus-point wins from five games.
But club president Brendon Hollows said the change of strip was a nod to its past.
"We have an annual heritage day," he said.
"But every four or five years we try to make it a bigger celebration for it and it coincides with a hundred years at Miller Park."
The Green Island Rugby Club was established in 1884 but played in various locations before settling on a permanent base at Miller Park in 1921.
"Back in the day we played in green and black hoops. But straight after World War I funds were reasonably tight and apparently grey was the easiest colour to buy."
Old team photos reveal a hotchpotch of "grey polo shirts, T-shirts and all sorts of things".
To recognise that history the team will play in grey jerseys, to be auctioned afterwards to raise money for the club.
Green Island is hosting Kaikorai (18 points) and three trophies are at stake.
The game doubles as the Speight’s Jug final and Green Island will also be trying to defend the Speight’s Shield — club rugby’s version of the Ranfurly Shield. The final piece of silverware is the Ken St Clair-Newman Trophy, which is contested annually between Green Island and Kaikorai.
Hollows hopes the event will attract about 250 people.
"We booked in this day ages ago and it just happens that all these things coincided, so it has turned out to be a big event and a good event for the club."
In the other games, a resurgent Southern (12 points) will host Harbour (12 points) at Bathgate Park.
A loss for either side will be damaging for their playoff prospects.
Alhambra-Union (11 points) is at home to University (12 points). AU scored late to beat University during the first round.
It is another critical game which could help decide the make-up off the top six.
Taieri (14 points) and Dunedin (16 points) shapes as another important fixture. Dunedin is on a three-game losing streak, so will be hoping to bounce back at Peter Johnstone Park. And Taieri will be keen to produce a more complete performance after a patchy effort to date.
