Björn and Frida attending the opening of ABBA WORLD
From the Daily Mail:

So how can we tell them apart now?
ABBA's red-head shows off blonde
 locks as she launches ABBA World

We might not have known their unfamiliar Swedish names, but there was one easy way to tell the girls from ABBA apart - one was the blonde, the other the red-head.
But it seems that easy distinction has been lost.
Anni-Frid Lyngstad, once a vibrant red-headed beauty, is now blonde-haired like her former bandmate Agnetha Fältskog.

Anni-Frid showed off her bleached locks as she joined her former bandmate Björn Ulvaeus at the premiere of the ABBA World exhibition at Earl's Court in London.
 ABBA has sold 400 million records since its 1970s heyday and spawned the hugely successful stage and film musical 'Mamma Mia.'
Swedish organizers say the exhibition is almost a theme park dedicated to the band and will be 'a place for total interaction' with the band.

The exhibition tells ABBA's story in 25 rooms, displaying costumes, a recreation of the band's Polar Studios and even the helicopter on the cover of the 1976 album 'Arrival.'
The four former members of ABBA support the project and have donated memorabilia as well as recording interviews for the exhibition.
But while they were once close, all four of the the band members are rarely reunited.
Björn was once married to Agnetha, while Anni-Frid's ex-husband is fourth bandmember Benny Andersson.