About working spaces
A working space is the default profile for an image while you're editing it in Photoshop. It defines the color space in which you are editing an image.
Lab
Cannot be selected as a working space in the Color Settings dialog box. If you choose to edit a document in Lab mode, the document is converted to your monitor's color space prior to viewing and you will have limited editing capabilities. Because Lab encompasses the complete range of visible colors, for practical reasons it is better to work in color spaces (such as RGB and CMYK) that are smaller and more in line with the color spaces of devices that capture, display, or output images.
RGB working spaces
Are based on the RGB color model. Some RGB working space options are device-dependent (such as a monitor-profile-based working space) and some are device-independent (such as Adobe RGB, Apple RGB, and sRGB). It's probably best to use a device-independent working space for most image editing.
CMYK working spaces
Are device-dependent. While an RGB working space can be device-independent, CMYK working spaces are based on actual combinations of ink and paper. It's often best to use an RGB working space for editing your image in Photoshop and then use the appropriate CMYK profile to convert your RGB image to CMYK in preparation for printing. The data in the CMYK working space is the data that will be used to convert the image to CMYK. For more information on RGB and CMYK working spaces, see Choosing an RGB working space and Choosing a CMYK working space.
Photoshop gives you the option of embedding the profile of your working space when you save an image. Embedding a profile is recommended because it tells Photoshop and other color-management-savvy applications the color meaning of the numeric values in the document.
