Colour matching: why skin is hard to copy

Hold a hand up to a window and then to a lamp. The same skin looks different, and the difference is not only brightness. That small experiment is the whole problem of color matching in a facial prosthesis. A systematic review in the Journal of Indian Prosthodontic Society opens by calling color matching a challenge that has always faced maxillofacial prosthodontists, and concludes that it is a crucial step in making the prosthesis. This page explains why, and what is done about it.

Why skin is difficult to copy

The same review says that special techniques in tinting and coloring are required because of the heterogeneity of tones and shades in human skin, the illusion of depth, and the varying degrees of translucency. It credits five pigments found in the layers of the skin with its color: melanin, melanoid, reduced hemoglobin, oxyhemoglobin and carotene. A 2018 laboratory study in the Journal of Advanced Prosthodontics adds that accurate color and translucency matching under different lighting conditions is what gives a natural appearance, even when the shape of a prosthesis is not perfectly in harmony with the facial tissues.

Another complication is that color differs from person to person and from site to site on the same face. The systematic review describes studies on color matching as heterogeneous because the available techniques depend on skin translucency, skin tones and the populations studied. No single recipe or chart covers everyone.

Color inside the silicone, and color on top of it

Two broad ways of adding color appear throughout the literature. Intrinsic coloring mixes pigment into the silicone before it cures, so the color runs through the material. A 2022 study in the International Journal of Biomaterials notes that intrinsic color can come from precolored silicone supplied in different shades, or from hand-coloring with pigments. Extrinsic coloring is applied to the surface afterward. The Queensland University of Technology review of soft-tissue prosthetics says the material has to be translucent like natural skin and take intrinsic staining, and also be able to be custom colored on the surface for realistic detail. One group cited in that review sealed surface colors with silicone adhesive and then finished the prosthesis with a matting dispersion liquid to reduce gloss.

In practice, makers layer and blend, using depth to imitate how light moves through skin. Which approach holds up better is a research question. The 2024 review of silicones in the journal Materials reports that ceramic pigments are generally recognized as ensuring greater color stability, while organic pigments and external makeup pigments are more prone to change.

The light problem

Skin color changes with lighting, and so does a prosthesis, but the two do not always change together. This is the effect called metamerism: a 2022 study in PLOS ONE defines instrumental metamerism as when two colors matched under certain conditions differ greatly under others. The idea is old in practice. The Smithsonian Magazine feature on the First World War mask studios quotes Grace Harper, chief of the Bureau for the Reeducation of Mutilés, on the difficulty: "Skin hues, which look bright on a dull day, show pallid and gray in bright sunshine, and somehow an average has to be struck." The same article reports that the sculptor Anna Coleman Ladd painted each mask while the man was wearing it, to match his own coloring as closely as possible. You can read how the story unfolded on the timeline.

Modern studies show the problem in numbers. The PLOS ONE team photographed silicone specimens in two windowed and two windowless clinics and took 432 measurements. They found significant color variations in the specimens owing to natural ambient light, and that image-calibration methods removed some of the differences in some settings but not in others. The takeaway for a lay reader is modest: a color judged in a clinic is judged in that clinic's light.

Instruments and recipes

The systematic review notes that the traditional approach is chairside visual trial and error, and that skin shade guides, spraying and tinting techniques, tattooing methods and even commercial cosmetics were developed over time. Compared with those, it says instrumental colorimetric or photometric techniques were noted to give more quantitative, reliable and consistent assessments of color under controlled conditions. Spectrophotometers and colorimeters are believed to have improved the efficiency of matching.

The 2018 study from the Journal of Advanced Prosthodontics tested a computerized system that turns a skin measurement into a pigment recipe. In laboratory specimens it found the system reliable for color and translucency across a range of skin colors. It also notes shortcomings of earlier attempts, including problems measuring translucent skin with a spectrophotometer and using industrial software that was not designed for skin. A protocol for a clinical trial published in Pilot and Feasibility Studies says that digital skin color measurements and colorant recipes may help the objectivity and efficiency of color matching, which is a hope rather than a finding. How measurement fits into the wider digital workflow is covered on digital design and 3D printing.

Why a good match does not last

Even a good match changes. The 2024 Materials review reports that the main reason for replacing a prosthesis is discoloration and mismatch with the surrounding tissues, and that ultraviolet exposure chemically transforms organic pigments. Cleaning matters too. The International Journal of Biomaterials study found that both the intrinsic coloring technique and the disinfection method significantly affected color stability over a simulated year of cleaning. Cambridge University Hospitals tells its patients that its prostheses should last 12 to 18 months because ultraviolet light and skin oils fade the colors.

These are reasons color is reviewed at follow-up and reasons prostheses are remade, not signs that something has gone wrong. For the materials involved, see what a facial prosthesis is made of. For day-to-day handling, see care and cleaning.

What the evidence cannot yet say

The sources opened for this page do not claim a perfect match under every light, and several of the studies are laboratory tests on silicone samples rather than on people wearing prostheses. The systematic review describes the studies it covers as heterogeneous, which makes direct comparison difficult. Anyone wearing a prosthesis, or considering one, should talk about color with their own prosthetist, who can see their skin in person. Living with a prosthesis that looks different in different rooms is part of what people describe on the page about living with a facial prosthesis.

In brief

Skin is translucent, uneven and lit differently everywhere, so matching it in silicone relies on layered pigments, surface coloring and, increasingly, measurement tools. Light, ultraviolet exposure and cleaning change the match over time, which is one reason prostheses are replaced. Research on digital color matching is promising but still mostly laboratory-based.