It could be a combination of things.
I haven't been in one BIR kitchen and found deggi mirch (too expensive). Instead they tend to use the standard hot (usually Rajah or similar) chilli powder, which is more orange because it contains far more seeds, and mix it with paprika to calm it down a bit and give it a more red colour.
Next suspect is tomato puree (diluted). While there are some dishes on the menu that don't include it, you'll find it in many BIR curries to some extent or another.
And there may well be a touch of red food colouring involved - especially if the chef has added a little red masala sauce to the dish. (Red food colouring is found in the commercial pastes used to make the masala sauce).