Defining spot color inks in Photoshop

Spot colors are special premixed inks used to replace or supplement process color (CMYK) inks. Each spot color requires a special plate at the time of printing. (Because the varnish also requires a separate printing plate when printing, it is also considered a spot color.)

Assign a spot color to the image using the following method:

To apply a spot color as a hue to the entire image, convert the image to "two-tone" mode and apply a spot color on one of the two-tone plates. You can use up to four spot colors, one for each plate.

To use a spot color for a specific area of ​​an image, you must create a spot color channel. The spot color channel lets you add and preview spot colors in the image. The following guidelines will help you use spot color channels:

If you need sharp-edged, spot-colored graphics and discard the images below, consider creating additional artwork in the page layout or illustration application.

You can create a new spot channel or convert an existing alpha channel to a spot channel.

As with the Alpha channel, spot channels can be edited or deleted at any time.

Spot colors cannot be applied to a single layer.

Spot colors are embossed on the top of the fully composite image.

If you print an image containing a spot color channel to a composite printer, the spot color is printed as an additional page.

You can combine a spot color channel with a color channel to separate the spot color into its color channel component. Merging Spot Channels lets you print a single-page proof of a spot color image on a desktop printer.

The name of the spot color is printed on the color separation page.

Each spot channel is stamped in the order it appears in the channel palette.

Unless in Multi-Channel mode, you cannot move spot channels above the default channel in the Channels palette.

To move spot channels in the channel palette so that they print before other plates, turn the image into "multichannel" mode.
Source: HC Network Printing Industry Channel