Short answer
Water itself absorbs light weakly in the red through vibrations of its molecules, and over metres of water that absorption removes red, then orange, then yellow. Blue survives furthest, which is why clear water and deep scenes look blue and why red objects look dark or black at depth. Dissolved matter and plankton add greens and browns in coastal and inland water.
Almost all colour in nature comes from electrons: pigments absorbing light, metals reflecting it. Water is the notable exception. Its O–H bonds vibrate, and although their fundamental vibrations absorb in the infrared, combinations and high overtones of those vibrations reach into the visible red. A four-quantum overtone lies near 698 nm, with further bands around 760 and 660 nm. Each absorbs only weakly, but over a few metres the effect accumulates: a 3 m tube of purified water looks distinctly blue when you look along it. Heavy water, whose heavier hydrogen shifts those vibrations to lower energy and out of the visible, is colourless — the proof that the colour comes from vibration rather than impurities.
Because absorption is strongest in the red and weakest in the blue, colours vanish in spectral order as the light path lengthens. Red goes first, then orange, then yellow and green; in clear ocean water blue penetrates furthest. NOAA notes that red light does not reach 100 m at all, so a red animal at that depth has nothing to reflect and appears black — effectively camouflage, which is why so many deep-sea animals are red. What counts is the total path: light travelling down to an object and then horizontally to a diver's eye loses red over both legs, so even a nearby object at modest depth is shifted.
Pure water tends to blue; real water rarely is pure. Phytoplankton add chlorophyll, which absorbs blue and red and pushes water towards green. Dissolved organic matter from rivers and peat absorbs strongly in the blue and gives tea-coloured or brown water. Over white sand in shallow tropical seas, light travels down and back through a short path, losing only a little red, and the sand reflects the rest: the result is the turquoise of lagoons. The site's nature section covers the colours of marine animals; this page is about the water between you and them.
Divers adapt to the blue-green light and underestimate how much colour has gone; memory of a reef is often more colourful than the unlit reef ever was at depth. Colour correction in cameras can rebalance moderate losses in shallow water, but once red is physically absent no processing can recover it — there is no red signal to amplify, only noise. That is why underwater photographers carry strobes or video lights: at short range, artificial light restores the full spectrum for the brief path to the subject and back.
| Approximate wavelength | Origin | Effect |
|---|---|---|
| about 760 nm | Vibrational overtone/combination band | Beyond the visible red; removes near-infrared |
| about 698 nm | Four-quantum stretch overtone | Absorbs deep red |
| about 660 nm | Higher overtone band | Absorbs red; the main contributor to visible blueness |
Why: Red and orange were absorbed along the light path before reaching the subject or the camera.
Fix: Get closer and use a strobe or video light; correction in software cannot restore absent red.
Why: The sample was judged dry in air; in the pool, a metre or more of water absorbs red and adds blue.
Fix: View a sample tile under water of similar depth before choosing.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
Water's intrinsic blue colour comes from weak absorption in the red by overtone and combination transitions of its vibrations, making it the only natural example of colour originating from vibrational transitions.
Source: Why is water blue?
FactStrong evidence
Heavy water (D₂O) is colourless because its corresponding vibrational transitions are shifted to lower energies, outside the visible.
Source: Why is water blue?
FactStrong evidence
In the ocean, blue light penetrates furthest, followed by green, yellow, orange and red; red light does not penetrate to 100 m, so red animals at depth appear black.
Caveat: Depths are for clear ocean water; coastal water attenuates much faster.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.