For white dhokla grind rice and urad dal into a coarse powder. Take the powder in a bowl and add two tablespoons yogurt and three fourth cup of warm water.
Mix thoroughly to make a batter of pouring consistency. Take care that no lumps are formed. Set aside to ferment for eight to ten hours.
For khaman dhokla take gram flour and turmeric powder in a bowl. Add remaining yogurt and approximately three fourth cup of warm water and mix well to avoid lumps. Set it aside to ferment for three to four hours.
Heat water in a pan. Place a ring in it. Take the fermented white dhokla batter and add a mixture of half tablespoon oil, half tablespoon lemon juice and one fourth teaspoon soda bicarbonate and mix thoroughly. Add salt and mix.
Heat water in a steamer. Grease a thali and pour the white dhokla batter till half full. Place it on the ring in the pan. Keep it in the steamer, cover and steam for five minutes Grind together coriander leaves, mint leaves, green chillies, one tablespoon lemon juice, salt, sugar and ganthia to a smooth paste.
Mix the remaining lemon juice, half tablespoon oil and turmeric powder in a small bowl. Add remaining soda bicarbonate and stir to mix.
Add it to the khaman batter and mix. Remove the white dhokla from the steamer and spread a thick layer of green chutney on it.
Pour the khaman batter over the chutney layer and steam again for ten to twelve minutes or until cooked. If a skewer inserted in the dhokla comes out clean it is done.
For tempering heat remaining oil in a pan and add mustard seeds. When the seeds splutter pour the tempering over the tri-coloured dhokla.
Cut it into squares and serve garnished coriander leaves and scraped coconut.