PDF (adobe.com)

Shift an out-of-gamut color to a printable color

Some colors in the RGB and HSB color models, such as neon colors, cannot be printed, because they have no equivalents in the CMYK model. If you select an out-of-gamut color, an alert triangle  appears in the Color panel or Color Picker.

 Click the triangle to shift to the closest CMYK equivalent (which is displayed in a small box by the triangle).