Two files can contain the same RGB numbers and show different colors if those numbers refer to different color spaces. The color space is the "vocabulary" that gives meaning to pixel values. Choosing it badly leads to dull photos on the web, prints with odd colors or gradients that break during editing. Let's look at the most widespread spaces and how to find your way among them.
What defines a color space
An RGB space is described by three elements:
- Primaries: the exact position of red, green and blue on a chromaticity diagram. The triangle they form is the gamut.
- White point: the color you get with all three channels at maximum. The most common is D65, representing average daylight; other spaces use D50, warmer, typical of graphic arts and printing.
- Transfer curve: the function linking the numerical value to light intensity (often loosely called "gamma").
Change just one of these elements and you get a different space, in which the same triplets of numbers correspond to different colors.
The main RGB spaces
| Space | Gamut | White | Typical use |
|---|---|---|---|
| sRGB | small, the common reference | D65 | web, most screens, sharing |
| Display P3 | wider, especially in reds and greens | D65 | modern phone and computer screens |
| Adobe RGB (1998) | wider, especially in greens and cyans | D65 | photography intended for print |
| Rec. 2020 | very wide | D65 | ultra-high-definition and HDR video |
| ProPhoto RGB | huge, includes invisible colors | D50 | photographic archiving and editing |
sRGB
This is the standard created in the mid-1990s to align monitors, operating systems and the web. Its gamut is modest, but it has a huge advantage: it is the default assumption. An image with no profile is treated as sRGB almost everywhere. For any content aimed at an unknown audience, it is the safest choice.
Display P3
It uses the primaries of the DCI-P3 cinema standard, but with a D65 white and the same curve as sRGB. Many recent phones, tablets and laptops cover most of this space. Photos taken on these devices are often saved in P3: more intense reds and greens that get compressed on an sRGB screen.
Adobe RGB
It extends the gamut mainly towards greens and cyans, an area where the inks of quality photo printers can go beyond sRGB. That is why it was long the preferred space for people who print. The downside: if an Adobe RGB image is displayed as if it were sRGB, it looks dull and desaturated.
Rec. 2020 and ProPhoto RGB
Rec. 2020 was created for next-generation television and has very saturated primaries; few screens cover it fully. ProPhoto RGB is even larger and includes "imaginary" colors, combinations the eye cannot see. It serves as a working space, so that nothing is lost while editing RAW files, not as a delivery format.
Wide gamut and bit depth
A wider space spreads the same numerical levels over a larger range of colors, so the "steps" between one color and the next are longer. With 8 bits per channel, a very wide space like ProPhoto can produce banding (visible stripes) in sky gradients. A practical rule:
- sRGB and Display P3: 8 bits are usually enough for delivery;
- Adobe RGB: 8 bits acceptable, 16 recommended during editing;
- ProPhoto RGB: use it only at 16 bits.
Which space to choose
A strategy that works in most cases:
- Shooting and development: process RAW files in a wide space at 16 bits, so you don't clip colors earlier than necessary.
- Retouching: stay in the wide working space.
- Web and social delivery: export to sRGB, or to Display P3 only if you know your audience will use compatible screens and color-managed software.
- Printing: deliver in the space the lab requests, or let the conversion with its profile handle the step.
- Archive: keep the 16-bit master in the wide space, alongside the exports.
Common mistakes
- Profile stripped on export: some compression tools remove metadata, including the color profile. A P3 image without a profile is read as sRGB and the colors change.
- Assigning instead of converting: assigning a profile changes how the numbers are interpreted (and therefore the visible colors); converting changes the numbers to preserve the colors. They are opposite operations, and confusing them is the most frequent mistake I see in files I receive.
- Judging on an uncalibrated screen: a wide-gamut monitor that has not been profiled shows sRGB files oversaturated.
In short
- A color space is defined by primaries, white point and transfer curve.
- sRGB is the default reference: the safe choice for web and sharing.
- Display P3 and Adobe RGB widen the gamut in different directions; Rec. 2020 and ProPhoto even more.
- Wide spaces need more bits to avoid banding.
- Assigning a profile and converting to a profile are different operations: do not confuse them.
