CMYK vs RGB
I do have a degree in graphic design.
There is no inherent advantage to working in CMYK with Photoshop, there are the disadvantages that you have mentioned. The LCD monitor can not display the full CMYK color gamut. So your argument that the color will be more accurate (less dull) is void. The modern digital press can accurately translate color information from wide gamut RGB to CMYK. There is no need to convert digital photos from RGB to CMYK.
I've been working in prepress for over 30 years. As a designer, do you really want to see a
wider gamut than the press is capable of producing? For the original poster, submitting an RGB file and hoping it will print in Black and Cyan is unwise - it won't happen unless the someone at the printing shop follows one of the suggestions already offered such as...
Good:
WITH INDESIGN (advantage: easy)
- Create custom color with 100k, 40c.
- Place grayscale tiff image in InDesign.
- With content tool, select image and change to custom color.
Better:
WITH PHOTOSHOP ONLY (advantage: more portable; no reliance on InDesign)
- Convert RGB to CMYK.
- Look at channels offering most detail. In my test, it was cyan.
- Copy and paste detailed channel to black.
- Clear magenta and yellow channels.
- Save as (CMYK) tiff.
Best:
PHOTOSHOP DUOTONE (advantage: absolute control over 2nd color midtone)
- Convert image mode to Duotone.
- Select "Duotone" from the menu in the subsequent dialog box.
- Click on the second square and select a Pantone color similar to cyan.
- Name the second color "cyan".
- Click on the curve and drag its center down. Refine curve for specific image.
- Save image as Photoshop EPS (with screening and transfer function options disabled).
Some print shops have an RGB workflow - meaning they convert RGB images to CMYK using custom ICC profiles before printing. If you obtain such profiles, it can be done before sending to the printer. This allows you to view the channels and see the results (assuming your monitor is calibrated). As far as throwing away data, you can always save an RGB original. Likely the image used in the document will be scaled to a specific size, resolution and cropping anyway. None of this is any consequence, anyway because, as already indicated, an RGB file will not work for the original question posted.
P.S. The highly respected David Blatner is begins by discussing formats photographers should save or deliver files. No one is disputing that RGB holds more color information and therefore is the preferred format for archiving originals. He then ends by suggesting submission of sRGB documents to printers. I respectfully, disagree. This particular RGB format is common in digital cameras and Web pages because it has vibrancy meant for computer displays. Adobe RGB (1998) has an even wider gamut and is often preferred by many professionals and recommended by Adobe. There are, however, other RGB formats - without even getting into 8-bit vs 16-bit color.
U.S. Web SWOP is for magazines. Typically, most CMYK printing jobs are sheetfed. If an ICC profile is not available from the printer, reasonably good results can be obtained by using a profile that describes the actual printing process. If you use one printing company that prefers RGB, more power to you!
Regarding the statement: "The modern digital press can accurately translate color information from wide gamut RGB to CMYK," it has not been established that the project in question is to be printed digitally. If a digital print is all that is required, a tinted RGB could be printed on a desktop printer. Here, I have a 12-color 44" digital printer that has a much wider gamut than my 4-color digital laser printer. The former prefers RGB because of so many inks but does well with CMYK. (Actually, all color images are converted to LAB and then RGB internally based on profile within the printer.) The laser printer has more predictable results with CMYK. Offset printing with ink typically uses 4 colors unless it is a special press for 6 or 8 colors, allowing for additional spot color and varnish.
