# xcolor black isn't black enough

I'm making a poster (using baposter) where I wish to include some microscopy images. To make the fluorescent signals stand out better, I changed the background of that cell to "pitchblack", which I have defined as

\definecolor{pitchblack}{cmyk}{0,0,0,1}


I've also tried this with

\definecolor{pitchblack}{rgb}{0,0,0}


However, with my microscopy images in the foreground, I find out that "black" isn't really black:

The baposterclass brings xcolor and tikz along for the ride, so I'm assuming my color issue is with those packages. So my question is, how I can make the "black" of the background frame black enough to match the background of these images?

Full code of the frame:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{center}
\renewcommand{\arraystretch}{0}
\begin{tabular}{@{}c@{}c@{}c@{}}
\raisebox{-2ex}{
\parbox[t]{0.25\textwidth}{\scriptsize\color{white} Representative volume renderings of HeLa cells imaged using TALEs
against the $\beta$-globin locus. Signals are approximately 0.3$\mu$m
in diameter.}} &
\imagetop{\includegraphics[width=0.3\textwidth]{dataimages/5_02_volume_view.jpg}} &
\imagetop{\includegraphics[width=0.3\textwidth]{dataimages/5_04_volume_view.jpg}} \\
\imagetop{\includegraphics[width=0.3\textwidth]{dataimages/5_05_volume_view.jpg}} &
\imagetop{\includegraphics[width=0.3\textwidth]{dataimages/5_07_volume_view.jpg}} &
\imagetop{\includegraphics[width=0.3\textwidth]{dataimages/5_10_volume_view.jpg}}
\end{tabular}
\end{center}
}


As a final note, the problem appears to be independent of the pdf viewer (i.e. it looks this way in evince,acroread,and gimp).

-
In case the information is useful for you: The black in the microscopy image is R:0 G:0 B:0 D:255 100% and the pitchblack is R:35 G:31 B:32 D:255 100%. – Stephen Oct 5 '12 at 18:06
That's interesting, but unsurprising. I guess my question really boils down to removing the minimal color that xcolor seems to insist on having. – Mark Kelly Oct 5 '12 at 18:09
Can you provide a full compilable minimal working example (MWE)? I get geometry errors after wrapping your frame code between \documentclass{baposter}\begin{document} and \end{document}. – Qrrbrbirlbel Oct 5 '12 at 18:23
Have you tried to use the black color already defined in xcolor? I mean something like (in tikz) \fill[black] (0,0) circle 1;... this should give you real black. If not, then there will definitely be some more examination and a MWE needed. – Benedikt Bauer Oct 5 '12 at 18:25
baposter loads xcolor by default with the option cmyk. All color models used now don't get it right. If you load the package option rgb and use \definecolor{pitchblack}{gray}{0} (or the equivalent RGB command, or the already defined black) it is pitchblack. – Qrrbrbirlbel Oct 5 '12 at 18:34

# Short answer: “K does not really stand for black”

\definecolor{pitchblack}{cmyk}{1 1 1 1}


This is a CMYK matter: baposter loads xcolor by default with the package option cmyk anymore.
Even passing rgb to xcolor doesn't help here.

Nonetheless, one should issue a test print, colors are not always what they appear to be.

A monitor uses the additive color model RGB, where your pixels consist of three sub-pixel. Additive means, that to have color one has to add light.
No output: black; full output: white.

A printer uses a subtractive color model like CMYK. Subtractive means removing light (when you add, for example, more cyan to your white paper, it will reflect less red).
No output: white (full reflection); full output: black.

### Why is black not black?

Good question!
The output depends on so many factors like paper, printer, toner, …

Again, check your printed output. Even in different PDF viewer the colors that are not CMYK(0, 0, 0, 1) or CMYK(1, 1, 1, 1) are represented by different RGB values.

## Examples

In the following MWE I provide three new “pitch”blacks to compare:

                         Name            CMYK             RGB
———————————————————————————————————————————————
pitchblack0    0   0   0   1    .35 .31 .32
pitchblack1   .6  .4  .4   1    .13 .12 .12
pitchblack2    1   1   1   1    .0  .0  .0
pitchblack3   .75 .68 .67 .9    .16 .14 .13
black          0   0   0   1    .35 .31 .32


### Code

\documentclass[tabular]{standalone}
\usepackage[cmyk]{xcolor}
\definecolor{pitch0}{cmyk}{0 0 0 1}
\definecolor{pitch1}{cmyk}{.6 .4 .4 1}
\definecolor{pitch2}{cmyk}{1 1 1 1}
\definecolor{pitch3}{cmyk}{.75 .68 .67 .9}
\newcommand*\blackbox[1]{\color{#1}\rule[-1ex]{3cm}{2em}}
\begin{document}\ttfamily
\begin{tabular}{lrrc}
Color  &           CMYK &         RGB &      Output       \\
pitch0 &        0 0 0 1 & .35 .31 .32 & \blackbox{pitch0} \\
pitch1 &     .6 .4 .4 1 & .13 .12 .12 & \blackbox{pitch1} \\
pitch2 &        1 1 1 1 &  .0  .0  .0 & \blackbox{pitch2} \\
pitch3 & .75 .68 .67 .9 & .16 .14 .13 & \blackbox{pitch3} \\
black  &        0 0 0 1 & .35 .31 .32 & \blackbox{black}  \\
\end{tabular}
\end{document}


### Output

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Thank you! I was about to give up and fix it by hand in gimp. – Mark Kelly Oct 5 '12 at 19:03
@MarkKelly I've updated my answer with, hopefully, helpful details. I might add that common-sense black seems to be pitch1 (.6 .4 .4 .1). In Adobe's Reader this is even rendered as RGB (0 0 0). If you can, issue a test print with my table and on of your space pictures. In the professional field, a print with pitch2 (0 0 0 0) may rejected, because the “full load” of all colors is used to plot printer's marks (such as crop and registration marks). – Qrrbrbirlbel Oct 6 '12 at 1:43
I fear that this would not be black but a very thick layer of ink when printed. :) – percusse Oct 6 '12 at 1:44