Krdytkyu

The MWE below does exactly what I want via the TypsetInBlueOnlyIfCurrentColorIsBlack macro:

TypsetInBlueOnlyIfCurrentColorIsBlack is intended to change the color of its parameter to blue, only if the current color is black. Otherwise, the current color is to be used.

Questions:

• I expected that xcolor would provide a macro to do this, but was not able to locate it. Thus, wondering if there any obvious problems with this approach, or is there an official interface that is provided.

References:

• This solution is adapted from How to get a textual representation of the current color?
  .

Code:

documentclass{article}

usepackage{xcolor}

usepackage{xstring}



%% At this point the default color is black so save that.

edefBlackColor{csnamestringcolor@.endcsname}



makeatletter

newcommand{@CurrentColor}{}% Make sure we are not using an existing macro

newcommand*{TypsetInBlueOnlyIfCurrentColorIsBlack}[1]{%

    edef@CurrentColor{csnamestringcolor@.endcsname}% Get current color

    IfStrEq{@CurrentColor}{BlackColor}{%

        {bfseriesttfamilytextcolor{blue}{#1}}% current color IS black

    }{%

        {bfseriesttfamily#1}%  current color is NOT black

    }%

}

makeatother



begin{document}

    Following should be in blue:

    TypsetInBlueOnlyIfCurrentColorIsBlack{blue}.



color{red}

    Following should be in red:

    TypsetInBlueOnlyIfCurrentColorIsBlack{red}.



color{blue}

    Following should also be in blue:

    TypsetInBlueOnlyIfCurrentColorIsBlack{blue}.

end{document}

An interface? Use expl3, of course!

With storecolor you can store the representation of the colors you need to do comparison with.

I define a generic comparison command and also yours as a special case.

At begin document each color in the list passed to storecolors and also black are converted in the internal representation and stored in a property list. This is for greater efficiency and not actually necessary: it saves several steps in the evaluation.

The main command converts the current color in the internal representation and compares it with the color given as the first argument (only named colors, but this could be generalized). If the test returns true, the color specified as second argument is used to print the third argument; otherwise no color change is performed.

documentclass{article}

usepackage{xcolor}

usepackage{xparse}



ExplSyntaxOn

NewDocumentCommand{perhapschangecolor}{mmm}

 {% #1 = color to compare with current

  % #2 = color to use if match

  % #3 = text

  peter_color_change:nnn { #1 } { #2 } { #3 }

 }

% the same but with black and blue preset

NewDocumentCommand{TypsetInBlueOnlyIfCurrentColorIsBlack}{m}

 {

  perhapschangecolor{black}{blue}{#1}

 }

NewDocumentCommand{storecolors}{m}

 {% black is always stored

  AtBeginDocument { peter_color_store:n { black,#1 } }

 }



prop_new:N g_peter_color_stored_prop

tl_new:N l__peter_color_current_tl



cs_new:Nn peter_color_store:n

 {

  hbox_set:Nn l_tmpa_box

   {

    clist_map_inline:nn { #1 }

     {

      color{##1}

      driver_color_pickup:N l_tmpa_tl

      prop_gput:NnV g_peter_color_stored_prop { ##1 } l_tmpa_tl

     }

   }

 }



cs_new_protected:Nn peter_color_change:nnn

 {

  driver_color_pickup:N l__peter_color_current_tl

  prop_if_in:NnTF g_peter_color_stored_prop { #1 }

   {% more efficient test

    str_if_eq:eeTF

     { l__peter_color_current_tl }

     { prop_item:Nn g_peter_color_stored_prop { #1 } }

     { textcolor{#2}{#3} }

     { #3 }

   }

   {

    hbox_set:Nn l_tmpa_box

     {

      color{#1}

      driver_color_pickup:N l_tmpa_tl

      tl_gset_eq:NN g_tmpa_tl l_tmpa_tl

     }

    driver_color_pickup:N l_tmpa_tl

    str_if_eq:eeTF { g_tmpa_tl } { l_tmpa_tl }

     { textcolor{#2}{#3} }

     { #3 }

   }

 }

ExplSyntaxOff



storecolors{red!60}



begin{document}



    Following should be in blue:

    TypsetInBlueOnlyIfCurrentColorIsBlack{blue}



color{red}

    Following should be in red:

    TypsetInBlueOnlyIfCurrentColorIsBlack{red}



color{blue}

    Following should be in blue:

    TypsetInBlueOnlyIfCurrentColorIsBlack{blue}



color{black}

    Following should be in pale red:

    perhapschangecolor{black}{red!60}{red!60}



color{red!60}

    Following should be in pale red:

    perhapschangecolor{black}{red!60}{red!60} 



color{blue}

    Following should be in blue:

    perhapschangecolor{black}{red!60}{blue}



    % blue has not been stored, but the test succeeds as well

    Following should be in pale red:

    perhapschangecolor{blue}{red!60}{red!60}



end{document}