Spot colors are special premixed inks used instead of, or in addition to, the process color (CMYK) inks. Each spot color requires its own plate on the press. (Because a varnish requires a separate plate, it is considered a spot color, too.)
If you are planning to print an image with spot colors, you need to create spot channels to store the colors. To export spot channels, save the file in DCS 2.0 format or PDF.
Note the following when working with spot colors:
For spot color graphics that have crisp edges and knock out the underlying image, consider creating the additional artwork in a page layout or illustration application.
To apply spot color as a tint throughout an image, convert the image to Duotone mode and apply the spot color to one of the duotone plates. You can use up to four spot colors, one per plate.
The names of the spot colors are printed on the separations.
Spot colors are overprinted on top of the fully composited image. Each spot color is printed in the order it appears in the Channels palette, with the topmost channel printing as the topmost spot color.
You cannot move spot colors above a default channel in the Channels palette except in Multichannel mode.
Spot colors cannot be applied to individual layers.
Printing an image with a spot color channel to a composite color printer will print the spot color at an opacity indicated by the Solidity setting.
You can merge spot channels with color channels, splitting the spot color into its color channel components.