This is a male butterfly of the Endemic species of Sri Lankan Lesser Albatross who comes under the family of Pieridae.
These exhibit migrations in large numbers and mud sip during these migrations.
Note : When the butterflies fly for long times, they also run out of body minerals like humans do the same when sweating. So they sip mud to gain the lost minerals directly.
This Albatross was clicked at the peak of Balumgala, Kadugannawa whch is a super windy place. But records say some of these prefer gloomy forests as well.