Eye colour comes from a combination of pigment and physics, not a single dye-like colour applied to the iris. Two mechanisms explain almost everything: melanin and light scattering.
The iris has two main layers: the stroma at the front, and the pigment epithelium behind it. The epithelium is densely pigmented in almost everyone. What actually determines visible eye colour is how much melanin sits in the stroma, and how that melanin — along with the stroma’s own structure — absorbs and scatters light.
High melanin density absorbs most light that enters the stroma, which is why brown and dark brown eyes stay a fairly stable, dense colour across most lighting — there’s simply less light left over to be affected by scattering or reflection.
A low-melanin stroma doesn’t have much to absorb light, so a different effect dominates: light scattering. Shorter (blue) wavelengths scatter more than longer wavelengths as light passes through the stroma’s fine structure — a Tyndall/Rayleigh-type effect, the same broad family of phenomenon behind why the sky looks blue. Blue eyes are blue for the same structural reason a clear sky is blue, not because of a blue pigment — the same underlying idea as structural colour in nature (feathers, butterfly wings), applied at the scale of the iris.
Grey eyes involve a similar low-melanin, scattering-dominated stroma to blue eyes, but a slightly different density or structure changes the balance of scattered wavelengths — which is also why grey eyes are among the categories most likely to be described differently by different observers, and to shift between grey, blue and green depending on lighting.
Hazel eyes typically combine moderate, unevenly-distributed melanin (contributing brown and gold) with a real light-scattering component (contributing green) in the same iris. Because the two mechanisms aren’t evenly mixed, hazel eyes commonly show visibly distinct zones — often a brown/gold centre with green toward the outer iris, or the reverse.
Amber’s distinctive gold/copper tone is generally attributed to a specific pigment balance in the stroma, without the strong scattering-driven green component that defines hazel — this is a less thoroughly sourced area of eye-colour science than the melanin/scattering story for blue and brown.
Because green depends on a balance between a small amount of pigment and a scattering effect, it’s unusually sensitive to anything that changes the light reaching and leaving the eye — ambient light colour, surrounding clothing and makeup colours, and pupil size all play a role.
Simultaneous contrast — the way a colour looks different depending on what’s next to it — applies to eyes too. Clothing and makeup near the eyes can make a given eye colour look more or less vivid without anything about the eye itself changing. This is the basis for the styling guidance included on each eye-colour page.
As the pupil dilates or constricts, the visible area and shape of the iris changes — which can shift how prominent a central ring (as in central heterochromia) or a patch (as in sectoral heterochromia) appears, without the underlying pigmentation changing at all.
Camera white balance, exposure and phone processing all shift colour in a photo, and screens cannot reproduce an eye’s real appearance exactly. Two genuine effects compound this: colour constancy (how the brain partly compensates for lighting when judging colour) and metamerism (two things matching under one light and not another). See lighting and photography for detail, and apparent colour for why Colourwise describes a range and its influences rather than one fixed value.
Sources: MedlinePlus Genetics (U.S. National Library of Medicine) and StatPearls (NCBI Bookshelf, NIH) — see individual eye-colour profiles for full source details.