Today is Gaudete Sunday, one of the two Sundays in the year when it has become customary to wear rose or pink vestments. This appears to derive from wearing the lightest and most festive violet set to mark the day as one of festivity in a penitential season.
When it came on this day to wearing choir habit Cardinals at some point began abandoning their seasonal penitential and mourning purple - other than the cappa magna - for pink or rose coloured cassocks, mantellettas and trains. This appears to have been practised mainly in Rome itself rather than elsewhere.
Last September the Liturgical Arts Journal had a post about the custom and featured a surviving example apparently from time of Pope Pius IX.
The illustrated article can be viewed at The Rose Coloured Vesture of Cardinals for Laetare and Gaudete Sundays