You got through the introduction on using Multichannel Mode, so you must be raring to learn more! If you recall, in the previous tutorial, I mentioned what happens when you have an RGB image open and you choose Multichannel from the Image > Mode menu. Remember to have the Channels palette open while you do this so you can see the RGB and composite channels change to CMY channels with no composite. To reiterate, the absence of a composite channel means Photoshop is no longer running in a color-managed environment. Let’s go a step further with this and manually generate a black channel. This will help balance the shadows and highlights.
Examine the three channels and choose one that has a nice, even distribution of shadows, midtones and highlights. Now launch the Calculations command which is located under the Image menu. For those who aren’t familiar with Calculations, this oldie was how compositing was achieved before Adobe introduced Layers into Photoshop. You had to have two images of equal dimensions opened up and then, using Calculations, blend the color channels together to create a mix of the two pictures. In our case, we’re going to use Calculations to blend our preferred channel to make a new 4th channel. With the Calculations window open, set both sources to the same channel – either cyan, magenta or yellow. (Feel free to mix different channels to see if that produces the even distribution of tones mentioned earlier.) By default, the blend mode is set to Multiply. I usually leave this value in place, but again, experiment to see if any other blend mode produces better results. If necessary, lower the opacity to dial back the effect. At the bottom of the window, make sure the Result drop-down menu is set to New Channel. Click Ok to make your new channel. This will show up with the name Spot Color 1 as a fourth channel in the Channels palette.
We’re almost there! Now, double-click on the thumbnail image of the spot channel to open the Spot Channels Options. In this little window, you should see the Name and Ink Characteristics, including a square color thumbnail. Double-click on the thumbnail and this will open up the Photoshop Color Picker window. Position the Picker window so you can still see the Options window. Change the CMYK values to C=0, M=0, Y=0, K=100. Notice that as soon as you enter the K value of 100, the name displayed in the Spot Options and the Channels palette changes to Black instantaneously. Click OK to complete the process of making the black channel.