RGB can be seen in the dark. CMYK can’t. RGB shines out at you. CMYK inks absorb some colours and reflect others.
Somebody wrote to me the other day whose website had lovely blue type on a black background. Fine. Unfortunately, the results when changed into CMYK and printed were dull, muddy and terrible.
How can this be prevented? Once converted to CMYK Photoshop will display its interpretation of how it will be printed. Further investigation should be made by using the Info palette and observing the CMYK values whilst hovering the mouse over certain areas of the image. Remember Photoshop can only give you an approximation both on screen and with its conversion values. Printed materials will never look the same as they do on screen.
The other thing you can do is change the CMYK values to make it look good. Look above. Picture number 1: How the website was, a black background with blue type. The blue was R80 G80 B252 (Hex: #5050fc).
Picture number 2: Look at the difference! That RGB colour when converted to CMYK is re-interpreted to a muddy, dull blue, C76 M70 Y0 K0. Almost illegible against the background black.
Picture number 3: The only possible solution. Lighten up the blue to a cyan dominated one, C64 M11 Y0 K0. Maybe not in the style of the website but at least punchy and legible.