Red Pigment can be obtained when processing Red Flowers in a Processor. 1 block of Red Flower will give 4 Red Pigments.
Red Flowers can be found growing on birch-like Ashenwood-trees in Grassland, Woodlands, Forests and valleys between Mountains, and they will grow slowly in time on Ashenwood Leaves, even in artificial biomes.
Red Pigment is a crafting-material used for crafting Red Beacons, Red Lamps (Wood Burning Lamps emitting red light), Red Fireworks, Red Stained Glass, Red Beds, Orange Block of Goo, Red Concrete, Red Mosaic Tile Wall, Red Adobe Floor, Red Adobe Roof, Red Adobe Wall, Red Carpet and Red Wood Wall.
As Pigment is merely a crafting-material, you cannot place Red Pigment not can you consume it; you cannot even put it into the quick-bar.