It simply comes from the colour of ginger. Its skin isn't really plain brown when cleaned up, but more of a yellow-brown-red mix of varying proportions. Also, ginger and redhead are technically different as they refer to different shades of red. Ginger is more a brownish red/orange.

Why is ginger a red color?

The pigment pheomelanin gives red hair its distinctive color. Red hair has far more of the pigment pheomelanin than it has of the dark pigment eumelanin. The genetics of red hair appear to be associated with the melanocortin-1 receptor (MC1R), which is found on chromosome 16.

What color is ginger naturally?

The natural coloring of fully-developed ginger is off-white or beige – any other hue means that food coloring was added. The one exception is if the root, or rhizome, was harvested at an earlier stage. Baby ginger is cream-colored and exhibits a bright pink at the tips from which its green stems arise.