PDF (adobe.com)

Choose a CMYK equivalent for a non-printable color

Some colors in the RGB, HSB, and Lab color models cannot be printed because they are out-of-gamut and have no equivalents in the CMYK model. When you choose a non-printable color in either the Adobe Color Picker or the Color panel, a warning alert triangle appears. A swatch below the triangle displays the closest CMYK equivalent.

Note: In the Color panel, the alert triangle is not available if you are using Web Color Sliders.
 To choose the closest CMYK equivalent, click the alert triangle  in the Color Picker dialog box or the Color panel.

Printable colors are determined by the current CMYK working space defined in the Color Settings dialog box.