10:30 amOff
sRGB is not always worse than ProPhoto RGB
We often hear that ProPhoto RGB is a much larger working color space than sRGB and that, therefore, it is preferable to use ProPhoto RGB's larger workspace.
In a way, this makes sense. But first, we have to define what is meant by "larger". This is based on the fact that workspaces are usually represented in the form of graphs that overlap each other, like this:
Graph representation of common workspaces
Unfortunately, this is partly misleading. The fact of the matter is that the different workspaces all have the same quantity of possible colors, they just don't contain the same distribution of colors (extremes and/or tone curve).
In other words, if you have an 8-bit sRGB image (8-bit per channel, that is), it will be able to define colors using 256 tones per channel, whereas if you have an 8-bit ProPhoto RGB image, lo and behold, it will be able to define colors using 256 tones per channel as well!
...Wait, what?
The difference is that the actual color each workspace means by, say "255,0,0" is not the same. It will represent a more saturated red in ProPhoto RGB than it will in sRGB, because the extremes of the ProPhoto RGB workspace (i.e: its gamut) represent more saturated colors (as we can see in the graph).
For simplicity's sake, let's say the two workspaces define red something like this (this scale is totally arbitrary, but you get the idea):
Scales of red in two different workspaces
As you can see, both only have 256 tonalities of red available, but the ProPhoto RGB's maximum red is richer than the sRGB's maximum red. This is all very nice, but this also means that the difference between each of the tones of the ProPhoto RGB workspace is greater, because it has a longer scale to cover with just as many tones.
This is not good, because not every subject has reds that are that extreme — it varies depending on the contents of the image, and very often, out of gamut colors are indiscernible, conspicuous regions of the image.
Suppose we have a subject where no color extends beyond what can be represented in the sRGB workspace (which is not a rare occurrence). Say, for the sake of the argument, that the full scale of the sRGB workspace was exploited, but no more, leaving 256 tones to express color. Using the previous (arbitrary) scale, the same range of tonalities in ProPhoto RGB would fall between approximately 0 and 215 — this time leaving only 216 tones to express the same colors.
As you can see, in that case, using ProPhoto RGB would actually be detrimental to the overall quality of the image. It follows that for the utmost quality, ProPhoto RGB should really only be used when tonalities extend beyond the extremes of a "smaller" workspace such as sRGB.
Please note that this is squarely a technical discussion and that one should still use ProPhoto RGB as a working color space all the time. The distinction is just not worth the hassle — it would demand too much work to constantly have to check each image for colors that fall out of gamut. Moreover, since one should always work in 16-bit color mode (or more), the problem is mitigated to a point where it really doesn't matter.