Materials are identified by how they handle light, not by their colour. A beginner paints leather, latex and polished metal with a base colour and a lighter version of it for the highlight, and all three come out looking like plastic. The corrections are small, specific, and they work at any level of stylisation.
Getting them right also does compositional work, since strongly reflective materials create bright shapes that direct the eye. Understanding how each behaves means those shapes can be placed rather than discovered.
Highlight shape is the main signal
A tight, small, very bright highlight reads as hard and polished. A broad, soft, dim one reads as matte. The size and softness of the highlight communicates surface roughness directly, and getting this right identifies a material before a viewer has looked at anything else.
The base colour matters much less than most people assume. A grey object with a tight highlight reads as polished; the same grey with a wide soft highlight reads as fabric. Spending attention on highlight behaviour rather than on base colour is the higher-return choice.
Latex and similar surfaces
Latex is dark in the body with an extremely tight bright highlight, often close to white regardless of the material's colour. It also stretches over form, so the highlight follows the underlying shape closely and breaks into separate bright shapes where the form changes direction.
Because it is stretched, it shows the form underneath more clearly than most materials, which makes it unforgiving of weak construction. It also creases in a distinctive way, with small tight folds rather than large soft ones, which is the material-behaviour argument that governs every kind of fabric.
Leather behaves quite differently
Leather scatters light rather than reflecting it sharply, so its highlight is broad and slightly diffuse, with a soft edge. It also has surface texture that breaks the highlight into a mottled pattern, and it creases permanently: a fold in leather stays as a mark long after the fold is gone.
Those permanent creases are what identify it and what make it read as an object with history, which is the wear argument in the piece on drawing bodies that show time. New leather and old leather look completely different, and choosing which is a characterisation decision.
Metal has no colour of its own
Polished metal reflects its surroundings, so what you paint is a compressed, distorted version of the room. Its darkest and lightest values are frequently both extreme within a small area, which is why metal painted as a smooth gradient looks like grey plastic every time.
The practical method is to decide what the metal is reflecting, usually a dark environment with one or two bright sources, and paint that. Even a crude version of it reads as metal, because the value structure is right, which is the fundamental point about value from the palette notes.
Edges carry the rest
How sharp the transition is between light and shadow varies by material. Hard shiny surfaces have abrupt transitions; soft porous ones have gradual ones. Getting the edge quality right is the second signal after highlight shape, and together the two identify almost anything.
This is also where stylised work can simplify without losing information. A flat-shaded rendering that keeps the edge quality distinctions still reads correctly, which is the summarisation principle from the piece on cel shading.
Materials against skin
Where a hard material meets skin, the skin deforms and the material does not, which is the deformation ratio described in the piece on props that interact with bodies. The value contrast at that boundary is usually strong, which makes it an attention point worth placing deliberately.
Bounce also matters. A bright reflective surface throws light onto nearby skin, and skin throws warm light back, which ties the two together and prevents the pasted-on look.
Making three materials look like three materials
Set the highlight size and softness first, give latex a tight highlight over stretched form, give leather a broad mottled one with permanent creases, paint metal as its reflections, and get the edge transitions right. Then check that everything shares the scene's light.
Characters here are fictional and unmistakably adult, as set out in the reader agreement.