Place the peeled raw shrimp in a large bowl and sprinkle over the lime juice and amchar masala. Mix until combined and cover with plastic wrap. Chill in the fridge for 1 hour.
Heat oil in a large pot over medium heat until shimmering.
Sauté onion, green pepper, red pepper and the white parts of the green onion to the hot oil and fry until beginning to soften, about 5 minutes.
Add in the green seasoning, curry powder, cumin and salt and fry for another 2-3 minutes until fragrant.
Add the water to the pot and turn up the flame to meadium-high heat. Bring the mixture to boil.
Add in the tomatoes and potatoes and simmer covered for 10-15 minutes until the potatoes are soft.
Add the seasoned shrimp and simmer for another 5 minutes until the shrimp turn pink and firm. Season with additional salt or scotch bonnet to taste.
Serve the shrimp curry with chopped culantro or cilantro. Hot cooked rice or buss up shut is a great accompaniment to this curry.