(Edit: this post discusses the previous versions of YRGBK.)
The first steps to making a printable color set from the YRGBK color set are to set the black point of each color channel, then adjust for dot gain. In the first, we are making sure that the darkest pixels representing each color are printing at 100%. The second step is to adjust for gain in the printing process.
The concepts apply to custom color separation in general, regardless of how the colors are generated. The YRGBK script merely provides a pretty good head start.
Black point, Dynamic Range, and Ink Coverage
By setting the darkest value to 100%, we increase the total printable range of each color channel. In addition to increasing the range of values we can represent, setting the black points also ensures good ink coverage in colorful areas of an image. This in turn reduces the amount of pressure needed to get a good print through a screen, improving both detail and dynamic range on press.
To set the Black point, select a spot color channel
• From the menu choose: Image/Adjustments/Levels
• Move the Black slider to the first set of pixels in the histogram
With the Black point set, the next step is to adjust for dot gain in the color channel. By applying a curve to reduce the density in the mid range, we can compensate for the gain that occurs on press. A standard adjustment is generally required, although making a custom adjustment for the particular image is best.
(Before making a dot gain adjustment, be sure to set the spot color setting, in the color settings menu, to the correct dot gain for your print setup. This setting modifies the spot color channel preview when more then one color channel is visible.)
To adjust for dot gain:
• Select a spot color channel
• Open Curves from the menu: Image/Adjustment/Curves
• Click on the center (50%) of the diagonal line and move it down to the 30% range.
• This adjustment can be customized to suit the image, spot color, and black point setting.
Setting a custom color
Increasing the value of a channel increases the amount of color printed. The color of a spot color channel should also be considered when setting the pixel density. Using a less saturated, printable ink color compensates for an increase in color density.
• The Use ink colors option in the YRGBK script sets the color channels to printable ink colors.