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copied!<p>I value ImageMagick in general very much -- but don't trust <code>convert</code> to count the colors correctly with the command you're using...</p> <p>May I suggest a different method to discover if a PDF page uses color? It is based on a (relatively new) Ghostscript device called <code>inkcov</code> (you need Ghostscript v9.05 or newer). It displays the ink coverage of CMYK for each single page (for RGB colors, it does a silent conversion to CMYK internally).</p> <p>First, generate an example PDF with the help of Ghostscript:</p> <pre><code>gs \ -o test.pdf \ -sDEVICE=pdfwrite \ -g5950x2105 \ -c "/F1 {100 100 moveto /Helvetica findfont 42 scalefont setfont} def" \ -c "F1 (100% 'pure' black) show showpage" \ -c "F1 .5 .5 .5 setrgbcolor (50% 'rich' rgbgray) show showpage" \ -c "F1 .5 .5 .5 0 setcmykcolor (50% 'rich' cmykgray) show showpage" \ -c "F1 .5 setgray (50% 'pure' gray) show showpage" </code></pre> <p>While <strong>all</strong> the pages do appear to the human eye to not use any color at all, pages 2 and 3 do indeed mix their apparent gray values from color.</p> <p>Now check each page's ink coverage:</p> <pre><code>gs -o - -sDEVICE=inkcov test.pdf [...] Page 1 0.00000 0.00000 0.00000 0.02230 CMYK OK Page 2 0.02360 0.02360 0.02360 0.02360 CMYK OK Page 3 0.02525 0.02525 0.02525 0.00000 CMYK OK Page 4 0.00000 0.00000 0.00000 0.01982 CMYK OK </code></pre> <p>(A value of 1.00000 maps to 100% ink coverage for the respective color channel. So <code>0.02230</code> in the first line of the result means <code>2.23 %</code> of the page area is covered by black ink.) Hence the result given by Ghostscript's <code>inkcov</code> is exactly the expected one: </p> <ul> <li>pages 1 + 4 don't use any of C (cyan), M (magenta), Y (yellow) colors, but only K (black).</li> <li>pages 2 + 3 do use ink of C (cyan), M (magenta), Y (yellow) colors, but no K (black) at all.</li> </ul> <p>Now let's convert all pages of the original PDF to use the <code>DeviceGray</code> colorspace:</p> <pre><code>gs \ -o temp.pdf \ -sDEVICE=pdfwrite \ -sColorConversionStrategy=Gray \ -sProcessColorModel=DeviceGray \ test.pdf </code></pre> <p>...and check for the ink coverage again:</p> <pre><code>gs -q -o - -sDEVICE=inkcov temp.pdf 0.00000 0.00000 0.00000 0.02230 CMYK OK 0.00000 0.00000 0.00000 0.02360 CMYK OK 0.00000 0.00000 0.00000 0.02525 CMYK OK 0.00000 0.00000 0.00000 0.01982 CMYK OK </code></pre> <p>Again, exactly the expected result in case of succesful color conversions! (<em>BTW, your <code>convert</code> command returns <code>2</code> for me for both files, the [original] <code>test.pdf</code> as well as the [gray-converted] <code>temp.pdf</code></em> -- so this command cannot be right...) </p>

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