SRGB: the colour profile for the web
Web designers: Use only sRGB for editing and saving all images intended for publication to the internet.
sRGB (“standard” RGB) has been established in an attempt to create a standard for the typical computer monitor. It is the default colour space of most monitors and effectively the recommended color space for the WWW. It is the colour space the Photoshop’s Save for Web saves “in”.
If you have your machine set up in a different colour space (e.g. Adobe RGB) and have a colour profile set up to anything other than sRGB you could get unexpected results when exporting via “Save for Web” or simply by “Save as” commands. Most often the colours in the “Save for Web window will appear wishy-washy or plain wrong, especially in reds – so called “colour shift”. You could also be designing with wrong assumptions about the colour dynamics within your layout.
The colour shift is caused by differences in gamut* range and gamma** settings between colour spaces. Remember sRGB is the colour profile of the Web and it is sRGB that Photoshop will use to render image. If your working RGB is Adobe RGB and not sRGB, you’ll get colour shift when saving for web – because Adobe RGB has much wider gamut.
*Gamut: a complete extent or range of the colour spectrum in a given colour space.
**Gamma setting: it tells the computer where the middle point between black and white is located. It determines contrast dynamics in your display (this is what makes Mac monitors appear lighter than PC monitors).