Mix besan in buttermilk, make a batter, stir it so that no lumps remain. Dissolve tartric acid in 2 tablespoon water. Add this water to the batter. Add haldi, salt and Eno to it.
Divide batter into 3 equal parts. Add red food color in one part and green food color to other. Put all three batters in a greased pan. Steam it in a pressure cooker on high heat for about 5 minutes and medium low heat for 15 minutes.
For tadka: Heat oil in a pan, add rai seeds to it when they begin to crackle, add kari patta and green chillies to it. Cook for few seconds and add 1/2 cup water to it. Add sugar, salt and pepper powder in the water. When green chillies are soft pour this tadka on the Dhokla.
Steam the dhoklas in a pressure cooker (no whistle) with a layer of water.