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With your color range still selected, make a new spot channel by pressing on the Channels menu in the top right (the 4 lines). Press "New Spot Channel."
After you have made the channel, you can also draw on or edit (e.g. erase) the channel to add more white, or take some away. Make sure to hide (press the eyeball) all of the other channels when doing this.
4A. Spot Channel Parameters
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Solidity: I usually put this between 40% and 70%. This is essentially the white's opaqueness.
4B. Optional: Edit your channel
After you have made the channel, you can also draw on or edit (e.g. erase) the channel to add more white, or take some away. Make sure to hide (press the eyeball) all of the other channels when doing this.
5. Save as Photoshop PDF.
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