Correcting Colors in CMYK and RGB
Although you can perform all color
and tonal corrections in RGB mode and most adjustments in CMYK mode,
choose a mode carefully. Avoid multiple conversions between modes,
because color values are rounded and lost with each conversion.
Don’t convert RGB images to CMYK mode if they are meant for on-screen
display. For CMYK images that are separated and printed, do not
make color corrections in RGB mode.
If you must convert your image from one mode to another, perform
most of your tonal and color corrections in RGB mode. You can then
use CMYK mode for fine-tuning. The advantages of working in RGB
mode are:
RGB has fewer channels. As a result, your computer uses
less memory.
RGB has a wider range of colors than CMYK, and more colors
are likely to be preserved after adjustments.
You
can soft proof colors to see an on-screen preview of how your document’s colors
will look when reproduced on a particular output device. See About soft-proofing colors.

You can edit an image in RGB
mode in one window and view the same image in CMYK colors in another
window. Choose Window > Arrange > New Window
For (Filename) to open a second window. Select the Working CMYK
option for Proof Setup, then choose the Proof Color command to turn
on the CMYK preview in one of the windows.