Usually yellow in colour but recipes vary by village.
Kokum is dried/smoked Tamarind(A souring agent)
http://vimmysrecipeworld.com/fish-with-coconut-gravy/
http://www.kurryleaves.net/2013/11/kerala-fish-curry-nadan-meen-curry.html...
I cook my own Bengal mustard fish curry or Kerala fish curries from scratch. Good restaurants are so thin on the ground I dont bother.
Have Vindaloo at Welfarespoons to mask the "meat", Favourites are Prawn Puree and lamb bhuna but havent been to Indian in years.