At the vegetable market, Tanmay sees a bunch of red amaranth (lal saag) with deep red leaves

Why

Gargi's reasoning has a sensible start: chlorophyll is green, and this leaf doesn't look green. But what we see is only the colour that wins, not every colour that is present. Leaves like lal saag contain plenty of green chlorophyll and also a large amount of red pigment, and the red masks the green. Red amaranth is not a parasite and doesn't feed on other plants — it grows on its own in ordinary soil and, like any green plant, makes its food by photosynthesis in its leaves. Test: when lal saag is boiled, the red pigment leaks out and turns the water pink-red; and a red leaf kept in sunlight, then decolourised and tested with iodine, turns blue-black, which shows that starch was made in it. Tanmay is right: red leaves still have chlorophyll and still photosynthesise; the green is simply hidden under the red.