I have to do this often for our newsletter. Here is what you do.
First ALWAYS work on a copy of the original file!
1. Open the grayscale airplane image and do whatever edits/adjustments you need to do.
2. Select the portion of the image you want to stay grayscale.
3. Decide how much you want the 'jet stream' to overlap the airplane. Then Select> Modify> Contract by that much (For a soft edge overlap do a Select> Feather 1-2 pixels less than your Contract) and Select> Inverse.
4. Open your Channels Pallet. Click on it's Options arrow and select 'New Spot Channel'. Your selection will become a Spot Channel.
5. Click on the Ink Characteristics> Color box to select/create your 'jet stream' color, name the Spot Color and adjust the Solidity % for ink coverage (for one color use 100%, overprinting of multiple channels check with your printer, but usually add channels to total of 90-100%).
6. You can now see the two channels approx. how they will print. To see each channel alone as a 'plate', click off one of the channel's eyes to see the other. You can jump between them by clicking on each channel's name. In this view you can also edit each channel individually.
Save as a DCS 2.0 EPS (Single File with Color Preview), Tiff, Photoshop PDF or Photoshop PSD file.
See easy and fun. I hope this helps!
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Chadoe