Loading the color package changes the default color of the fonts

Question

I have three simple files which load colour-related packages although they don't attempt to use any fancy colours. These are file col.tex:

\documentclass{minimal}
\usepackage{color}
\begin{document}
ABC
\end{document}

file col-black.tex:

\documentclass{minimal}
\usepackage{color}
\begin{document}
\textcolor{black}{ABC}
\end{document}

and file xcol.tex:

\documentclass{minimal}
\usepackage{xcolor}
\begin{document}
ABC
\end{document}

I latex and then dvips (with dvips(k) 5.994) each of them, and I look at the postscripts. I was expecting to get "ABC" in black in all cases, as if I had not loaded any colour package at all. However, when visualizing the files with evince, this is not the case for file col.ps, I get the "ABC" in a very dark grey, but clearly not black. If I ps2pdf the postscripts the differences are even more clear using a large magnification in acroread.

If I diff the three postscript files, apart from three mentions in each of them to the dvi file names, a couple of lines show differences: in col.ps:

%%Page: 1 1
TeXDict begin 1 0 bop Black Black 166 83 a Fa(ABC)p Black
Black eop end
%%Trailer

in col-black.ps:

%%Page: 1 1
TeXDict begin 1 0 bop Black Black 0 TeXcolorgray 166
83 a Fa(ABC)p Black Black Black eop end
%%Trailer

and in xcol.ps:

%%Page: 1 1
TeXDict begin 1 0 bop 0 TeXcolorgray Black 0 TeXcolorgray
166 83 a Fa(ABC)p 0 TeXcolorgray 0 TeXcolorgray eop end
%%Trailer

Am I doing anything wrong? Is this the expected behaviour of the color package? If it is, is there any way to set the default color in the header?

---

The problem seems to be with the definition of Black. In all files it is defined as

 /Black{0 0 0 1 setcmykcolor}

If I change the definition in col.ps to

 /Black{0 setgray}

I get the proper black I expected.

Answer 1

The choice of default colour is not really in the control of the color package, it is handled in the def file for the back end in use (dvips here. I'm not sure why its cmyk black isn't as black as rgb but anyway you can force it to use rgb black as the default by setting the colour in the preamble, compare the effect with or without the commented line.

\documentclass{minimal}
\usepackage{color}
%\definecolor{black}{rgb}{0,0,0}\color{black}
\begin{document}

 \rule{2cm}{1cm}

 \textcolor[rgb]{0,0,0}{\rule{2cm}{1cm}}

 \textcolor[rgb]{0,0,0}{\rule{2cm}{1cm}}



\end{document}

Answer 2

Your documents use different color models (and colors given in different color models): rgb and cmyk. Default color is a cmyk black. And black form color or xcolor is a rgb black.

With xcolor, you may convert color from a model to another. Here is an example to defined a cmyk black form rgb black:

\usepackage{xcolor}
\colorlet{black cmyk}[cmyk]{black}

With xcolor, you may choose a global color model:

\usepackage[rgb]{xcolor}

Here is a complete example to show the differences with black, green and cyan. In this example, each row of a big square contains three small squares: the color from xcolor, the color from xcolor converted to cmyk model and the color form xcolor converted to rgb model. The first row uses the global model natural (no conversion), the second row uses the global model rgb and the third row uses the global model cmyk (note: black and green from xcolor are in rgb color model while cyan is in cmyk color model).

\documentclass{minimal}
\usepackage{xcolor}
\usepackage{graphicx}
\colorlet{black cmyk}[cmyk]{black}
\colorlet{black rgb}[rgb]{black}
\colorlet{green cmyk}[cmyk]{green}
\colorlet{green rgb}[rgb]{green}
\colorlet{cyan cmyk}[cmyk]{cyan}
\colorlet{cyan rgb}[rgb]{cyan}

\def\myrule{\rule[-5mm]{1cm}{1cm}}
\def\mytest{
\textcolor{black}{\myrule}%
\textcolor{black cmyk}{\myrule}%
\textcolor{black rgb}{\myrule}
\textcolor{green}{\myrule}%
\textcolor{green cmyk}{\myrule}%
\textcolor{green rgb}{\myrule}
\textcolor{cyan}{\myrule}%
\textcolor{cyan cmyk}{\myrule}%
\textcolor{cyan rgb}{\myrule}
}
\def\mytitle{
  \parbox{2.5cm}{Colors}
  \parbox{1cm}{\centering\rotatebox{90}{\texttt{black} (default)}}%
  \parbox{1cm}{\centering\rotatebox{90}{\texttt{black cmyk}}}%
  \parbox{1cm}{\centering\rotatebox{90}{\texttt{black rgb}}}
  \parbox{1cm}{\centering\rotatebox{90}{\texttt{green (default)}}}%
  \parbox{1cm}{\centering\rotatebox{90}{\texttt{green cmyk}}}%
  \parbox{1cm}{\centering\rotatebox{90}{\texttt{green rgb}}}
  \parbox{1cm}{\centering\rotatebox{90}{\texttt{cyan} (default)}}%
  \parbox{1cm}{\centering\rotatebox{90}{\texttt{cyan cmyk}}}%
  \parbox{1cm}{\centering\rotatebox{90}{\texttt{cyan rgb}}}
}
\begin{document}
\mytitle

\selectcolormodel{natural}
\parbox{2.5cm}{Model \texttt{natural}:} \mytest

\selectcolormodel{rgb}
\parbox{2.5cm}{Model \texttt{rgb}:} \mytest

\selectcolormodel{cmyk}
\parbox{2.5cm}{Model \texttt{cmyk}:} \mytest

\end{document}