By experiments on animals, Holth has decided that the best pigments to use in tattooing a representation of the iris on the cornea are as follows: for black, lampblack; for red, cinnabar; for brown, a mixture of the above, oxide of iron, terra de Sienna, or sepia; for yellow, red terra de Sienna; for blue, ultramarine.
Reference: Holth S. Tattooing the cornea in colors. Arch Ophthalmol. 1905:209.
A look at the past . . .. Arch Ophthalmol. 2005;123(12):1690. doi:10.1001/archopht.123.12.1690