A great way to save on your printing costs
is to use two inks instead of four. You can also produce
some incredibly striking images, and some cool effects, for
print, Web, and artistic purposes. But which color mode is
right for your project, Duotone or Multichannel? Let's do a
comparison.
Duotone color mode takes a grayscale image
and adds an accent color. If you're starting with CMYK or
RGB, you'll have to use the command Image> Mode> Grayscale
first. You can then access Image> Mode> Duotone. Duotone, by
the way, is sometimes used as a generic term that also
covers monotone (one color), tritone (three colors), and
quadtone (four colors). You choose your colors in the
Duotone dialog box.
By clicking on a color swatch you open the
Color Picker. Typically when creating a Duotone, you'll find
using Custom colors simpler. When the Color Picker opens,
click on the Custom button. Select the type of ink from the
pop-up menu (dependent, of course, on your print job's
specs), and choose a color. In this example, we'll create a
sepia look by using Pantone Process Yellow. (To the left of
the color swatch is the Duotone Curve button. We'll explore
what you can do with this option later.) The Overprint
Colors button at the bottom is used primarily with tritons
and quadtone images. It gives you control, to some degree,
over how inks will interact. Note that you can re-open the
Duotone dialog box at any time to make changes by simply
using the command Image> Mode> Duotone.
When we compare the grayscale and duotone
images, you can see that Photoshop has changed the hue of
all the pixels, rather than perhaps just adding some colored
accents.
Let's look a little more closely, using
Color Samplers and the Info palette.
When comparing the two Info palettes, notice
first that the duotone's color values are shown as "1" and
"2," which represents the inks selected in the Duotone
dialog box. Next, notice that the black percentages for each
spot are unchanged – Photoshop simply added an equal amount
of the second color to the pixel's color value.
Now let's look at the Channels palette to
see what a duotone is actually all about.
You'll see that there is but a single
channel, despite the fact that we're using two colors in the
image. In Photoshop, rather than controlling the placement
of ink by editing a color channel, we use the Duotone Curves
dialog box (mentioned earlier). Open the dialog box by using
the menu command Image> Mode> Duotone, then clicking on the
box to the far left of the ink you want to edit.
If you work with Photoshop's Curves
adjustment (Image> Adjust> Curves), this box will look
somewhat familiar. The diagonal line represents how much ink
will be placed at given color values. Think of it as a
grayscale continuum, ranging from white to black. By
default, the ink is distributed proportionally for each
pixel value, from 0% to 100%.
You can change values by clicking and
dragging on the line or by entering numbers in the fields.
Let's see what happens when we make some adjustments.
As you can tell, you have a great deal of
control over how the ink is distributed according to pixel
value. Re-opening the Info palette shows what actually
happens to the pixels' colors.
Notice that Ink #2 (black) remains
unchanged. The first ink, however, is altered in direct
relationship between the percentage specified in the Info
palette and the value you assign in the curve. The
percentage of ink shown in the Info palette corresponds
directly to the percentage in the curve.
One further note on the construction of
duotones: You are not limited to "black and another color."
You can use any two colors you desire.
You can also extend the tonality of a
grayscale image by using black and a supplemental gray ink
Note that Photoshop let's you save duotone
images in a limited number of file formats.
In Round 2 of this battle, we'll explore the
Multichannel color mode.