As the climate in Sri Lanka is tropical and winter season is not expected, more light weight clothing and less warming clothes would be required. In the hill country you may need clothes like waterproof jackets.
If you visit the temples and other religious sites, the dress must cover shoulders and knees. When entering a such places, hats and shoes must be removed, and it is also advised to carry your own pair of socks to protect your feet as the floor can be rough at hot at certain places.
Depends on the tour you have selected, but in general, lightweight shoes or sandals are recommended, if you select a hiking tour in hill country consider taking leech socks with you.