Numbers in light and colour
Digital colour is built on 8-bit binary: three RGB channels (red, green, blue), each stored as 8 bits with 256 possible values (0-255), which is why 3, 8, 16, 24 and 255 keep appearing throughout colour and display technology.
These numbers are all encoding conventions, not physical constants — an 8-bit channel isn't the only way to store colour (10-bit and 12-bit exist for professional displays), it's simply the standard that won out for everyday screens and web colour. Colour temperature (measured in kelvin) and photometric brightness (lumens, lux) are physically real but conventionally-referenced quantities this hub keeps clearly distinct from the encoding numbers above.
The RGB colour model
Screens produce colour by mixing light from three channels — red, green, and blue — rather than mixing pigments the way paint does. Each channel is an intensity value, and every colour a screen can show is some combination of the three. This is why RGB is called an additive colour model: combining all three channels at full intensity produces white, the opposite of how mixing paint pigments works.
Why 8 bits, and where 255 and hex notation come from
Each RGB channel is standardly stored as 8 bits — a byte — giving 2⁸ = 256 possible values, numbered 0 through 255. Hexadecimal (base-16) notation writes each channel as two hex digits (since a two-digit hex pair covers exactly 0-255), which is why a full hex colour code like #FF8800 is always six digits: two digits each for red, green, and blue.
Some professional and HDR displays use 10-bit or 12-bit channels instead of 8-bit, for a wider range of colours than the standard 256 levels per channel — 8-bit remains the near-universal default for everyday web and consumer screens.
Encoding numbers vs physically measured light
RGB values, hex codes, and bit depth are all encoding conventions — ways of representing colour in binary, not physical properties of light itself. Colour temperature (measured in kelvin) and brightness (lumens, lux) are genuinely physical quantities, but the specific reference points commonly used (like "2700K warm white") are still conventional choices about which part of a continuous physical scale to label, not fixed mathematical facts the way 2⁸ = 256 is.
Numbers featured in this hub
Common questions
What do RGB numbers mean?
RGB describes a colour as three numbers — red, green, and blue channel intensities — each commonly ranging from 0 (none of that colour) to 255 (full intensity). Mixing all three at full intensity (255, 255, 255) produces white; all three at 0 produces black.
Why does RGB go from 0 to 255?
Each channel is standardly stored as 8 bits of binary data. 8 bits can represent 256 distinct values (2⁸), numbered 0 through 255 — so 255 isn't an arbitrary round number, it's simply the largest value an 8-bit channel can hold.
Why do hex colours use six digits?
A hex colour code like #FF8800 packs the same three RGB channels into hexadecimal (base-16) notation, using two hex digits per channel (since two hex digits can represent 0-255) — six digits total for red, green, and blue.
What does 24-bit colour mean?
24-bit "true colour" stores each of the three RGB channels as 8 bits (8 × 3 = 24), giving 256 × 256 × 256 — about 16.7 million — possible colours.
What does 2700K mean?
Kelvin (K) describes a light source's colour temperature — how warm (amber, lower K) or cool (blue-white, higher K) it appears. 2700K is a warm, incandescent-like white; 6500K is a cool, daylight-like white. This hub notes the concept; a dedicated kelvin profile isn't built yet (see docs/next-sprints.md).
What's the difference between lumens and lux?
Lumens measure the total light a source emits; lux measures how much of that light actually falls on a given surface (lumens per square metre) — the same bulb produces a fixed lumen output but very different lux depending on distance and surface area.