Colour Gamut

One reason it’s a good idea to edit in a colour space with a larger gamut than your target space is that your colour adjustments will have greater fidelity. If you limit your working space early, your colour adjustments and retouching work will be artificially limited, and in some cases this can cause banding. For example, if you adjust contrast by stretching the colours in a small range out to a larger range (as in Levels) then using 16-bit with ProPhoto gives you more information to stretch out, and therefore better quality. Think of colour fidelity as similar to image resolution.

Working with a larger gamut up-front also gives you more options later. In ten years, when we have better quality monitors, all the sRGB images will look flat compared to ProPhoto RGB images. The same applies now if you print to a higher-quality printer. Many layout artists have historically worked directly in CMYK (even less gamut than sRGB) space, but again, this is a device space, not a working space. If you try to print a CMYK image on a 6-color printer or show it on screen, it looks flat. Stick with a higher gamut (like Adobe RGB or better) for as long as you can.

Also remember that dynamic range is one of the important things that you get when you buy professional equipment. You’re not going to get truly good results out of any kind of cameraphone, for example.