Colour cosmetics are built from a small number of functional components, and the differences between products come from formulation choices that are more comprehensible than the marketing suggests.
The three parts
Pigment provides colour. Binder holds it together and to the skin. Vehicle carries it and determines application.
Everything else — preservatives, texture modifiers, treatments — sits around these.
The ratio between them determines whether a product is sheer or opaque, matte or luminous, long-wearing or easily removed.
Pigments
Iron oxides provide the reds, yellows and blacks that make up most skin-tone shades.
Titanium dioxide and zinc oxide provide opacity and whiteness, and both also absorb ultraviolet light, which is why they appear in sun protection.
Organic pigments and lakes provide the brighter colours that mineral pigments cannot achieve.
Pigment particles are frequently surface-treated — coated to change how they interact with oils and water — and this treatment does a great deal of the work in long-wear formulations.
Why foundations oxidise
A widely reported phenomenon with several contributing explanations.
Some pigments genuinely react with air and skin oils, shifting colour over hours.
More frequently, the apparent shift comes from the vehicle evaporating, concentrating the pigment, and from the product settling into the skin's own colour and oil.
Which is why testing on the jaw in daylight and waiting twenty minutes is more informative than an immediate assessment under shop lighting.
Coverage and finish
Coverage is pigment load. Higher pigment concentration means more opacity, which also means the product sits more visibly.
Finish is determined by particle shape and by the presence of light-scattering or reflective particles.
Matte finishes use absorbent powders that take up oil, which is why they can look dry on dry skin.
Luminous finishes use particles that reflect light diffusely, which is why they emphasise texture on skin that has any.
Neither is better and matching finish to skin condition matters more than matching coverage to preference.
Long wear
Achieved through film formers — polymers that create a flexible layer as the vehicle evaporates.
Which resists transfer and water, and which also means removal requires breaking the film, generally with oil or a dedicated remover rather than with water alone.
Silicones do a great deal of work here, providing slip on application and water resistance after.
The trade-off is that a film sitting on skin will emphasise any texture underneath, and it can feel occlusive.
The shade range problem
Foundation matching fails for a structural reason.
Skin colour varies on two axes — depth and undertone — and ranges historically expanded along depth while offering limited undertone variation.
Which meant many people could find their depth or their undertone but not both.
Ranges have expanded considerably, and the improvement has been in undertone coverage as much as in the number of shades.
Matching in daylight against the jaw, not the hand or wrist, remains the only reliable method, since those areas differ in colour from the face.
Expiry
Worth taking seriously for eye products specifically, where contamination risk is highest and consequences are worst.
Mascara has the shortest recommended period, generally a few months, because the wand carries bacteria back into a warm moist tube every use.
Powder products last far longer because there is no water to support growth.
Change in smell, texture or separation is a clearer signal than any printed date, and any eye product should be discarded after an eye infection without exception.
The honest summary
Formulation quality is real and it is mostly about pigment treatment, dispersion and the film system, none of which appear on packaging.
Which means testing is the only reliable method, and it is why the same person can find a cheap foundation that works perfectly and an expensive one that does not.
Application tools
Which affect the result more than most product differences do.
A brush deposits product on the surface and moves it around. A sponge absorbs some product and presses the rest into the skin.
Which means the same foundation applied three ways produces three finishes, and complaints about a product are frequently complaints about a technique.
Damp sponges absorb less product than dry ones, which is why the instruction to wet them exists.
Brushes and sponges harbour bacteria and should be washed regularly, and almost nobody does it often enough.
Primers
A category with mixed evidence and one clear function.
Silicone-based primers fill surface texture, creating a smoother base, which is a genuine physical effect.
Claims about extending wear are more variable and depend on the interaction between primer and foundation chemistry.
Silicone primer under water-based foundation frequently causes separation, which is the most common cause of makeup pilling and is entirely predictable from the chemistry.
Setting
Powder absorbs surface oil and reduces shine, and applied heavily it emphasises texture.
Setting sprays form a light film that reduces transfer, which is a real effect of modest magnitude.
Neither prevents the underlying skin from producing oil, which is what actually determines wear time.