The CIE chromaticity chart shows all colours available to the human eye, laid out flat. The curve shows all spectral colours, and the bottom dotted line shows our non-spectral pinks and purples.
It has to be said though that you are only looking at a representation of the chart, because your monitor likely cannot display all the colours your eye can see.
Given that monitors use only three base colours, they can only show a triangle’s worth of colours on the chromaticity chart, and never the whole horseshoe.
sRGB is probably the most common colour space for monitors, and it has been for a long time (1996). It has actually been so prevalent that it has become a problem: Modern monitors can often show colours outside of the sRGB triangle, but everyone on the web assumes sRGB, leading to you seeing over-saturated colours on better monitors, essentially punishing you for having a better monitor than the person who made the content.
The colour space problem is slowly being solved, with P3 monitors becoming more common, and Rec.2020 monitors starting to appear.
CSS got support for colour spaces beyond sRGB via the color() function around 2023:
.p3 {
color: rgb(255 0 0); /* graceful fallback */
color: color(display-p3 1 0 0);
}
.rec2020 {
color: color(rec2020 1 0 0);
}Print uses a subtractive colour model, meaning that mixing all the colours together produces black, and mixing none of them produces white. This is the opposite of monitors, which use an additive colour model, where mixing all the colours together produces white, and mixing none of them produces black.
CMYK also works with different primary colours than monitors — cyan, magenta and yellow — which means that the colours it can produce are different from the triangle of colours a monitor can produce. The result is that CMYK can produce some colours that monitors cannot, and vice versa.
Since mixing pigment is quite a bit different than mixing light, printing a photo on paper will never look quite the same as displaying it on a monitor. But nobody is printing photos anymore, right?