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Shine is a surface property. Light striking a hair shaft with a smooth, flat cuticle reflects specularly, producing a defined highlight band; light striking a lifted, chipped cuticle scatters, and hair reads as dull. Nothing about the interior of the shaft changes this.

A gloss works on that surface in two ways:

1. An acidic pH — typically 4.5 or below — which contracts the shaft and flattens the cuticle scales. This is the same mechanism as an acidic Hair Rinse, and it is where most of the shine comes from.
2. A film of dye and conditioning agents that fills surface irregularities, evening the reflection.

A clear gloss contains no dye at all and does only the first two jobs. That is a genuine product, not a con — smoothing the cuticle is most of the visible result.

Add direct dyes and the same treatment corrects tone: violet against yellow, blue against orange, green against red, or a warm gold or copper wash to enrich. That is the colour-wheel logic described at Hair Tint and Toning Shampoo.

Salon glosses may be demi-permanent — mixed with a low-volume developer, which makes them oxidative and longer-lasting; see Demi-Permanent Hair Colour. At-home glosses are usually direct dye only and last four to six weeks.

• Lighten. Nothing deposit-only can.
• Cover grey solidly. It tints grey translucently, which reads as blended rather than covered.
• Repair damage. It makes damaged hair look better by smoothing the surface, which is worth having and is not repair.
• Last. Four to six weeks for a direct-dye gloss, and less on porous hair.

Cosmetic — FDA primary category (07) hair colouring preparations for a tinted gloss; a clear gloss with no dye is closer to a conditioning treatment under category (06).

The regulatory split matters:

• Direct-dye glosses are generally subject to the ordinary colour additive requirements — approved for the intended use, certified colours from FDA-certified batches.
• Demi-permanent glosses containing oxidation dye precursors fall under the coal-tar hair dye exemption, which removes FDA's authority over their safety provided the prescribed caution and patch-test statement appears — and they carry the PPD sensitisation risk described at Permanent Hair Dye. A 48-hour patch test applies.

In the EU, hair dye substances are assessed by the SCCS and listed with limits in Annex III, with prohibited dyes in Annex II.

Clean, towel-dried hair — product residue blocks even deposit. Gloves; barrier cream at the hairline. Process for the stated time, checking early and often, because over-toning is the standard mistake and a violet gloss left too long on porous blonde hair goes lilac. Rinse in cool water.

Unopened, two to three years. Direct-dye glosses keep 12–24 months once opened if water and gloved hands stay out of the tub. Demi-permanent glosses must be used immediately once mixed and the remainder discarded.

• Hair Colour — the shelf this sits on, and the rest of it.
• Cosmetics — what the label is legally allowed to say, and the full catalogue.
• The rest of Colour — Permanent Hair Dye · Demi-Permanent Hair Colour · Semi-Permanent Hair Colour · Temporary Hair Colour · Hair Tint · Hair Chalk · Hair Colour Spray · Root Touch-Up.
• Regulatory statements here follow the FDA and EU sources listed on Cosmetics, read 16 August 2026.