Get decode array that turns black into specific color

Question

I have chosen a specific color that I am using for my letters. The color is used in headings and titles. I also have a black and white image that I want to use as a background, but I want it to have my chosen color instead of black. Is there a simple way to do this, without tweaking the parameters by hand?

MWE

\documentclass[12pt]{article}

\usepackage[top=0.78in, left=1in, right=1in, bottom=1in]{geometry}  
\usepackage{graphicx}
\usepackage[usenames,dvipsnames]{xcolor}
\usepackage{scrpage2}
\usepackage{background}
\usepackage{lipsum}

\colorlet{MyColor1}{RoyalPurple}
\newcommand{\HRule}{\rule{\textwidth}{1.5pt}}
\pagestyle{scrheadings}
\clearscrheadfoot
\chead{{\color{MyColor1} \textsc{\Huge{My Name} \\[-1.5pt] \Large{some tagline}}\\ \Large{\vspace{-8.5pt}} \HRule}}
\footskip = 0pt
\cfoot{\newcommand{\NL}{ $\ast$ }
{\color{MyColor1} \HRule \textsc{\footnotesize{\\ My address} \footnotesize{ \\ my.email@server.com \NL webpage.com \\ +44 123 456 789}}}}

\backgroundsetup{contents={\includegraphics[decodearray={MyColor1}, width=0.1\textwidth]{example-image}}, angle=0, position=current page.south, anchor=above, vshift=10pt}


\begin{document}
{\color{MyColor1}\Huge Title}\\[1cm]
\lipsum[1-5]
\end{document}

Answer 1

You can play with the blend modes offered by TikZ.

\documentclass[12pt]{article}

\usepackage[top=0.78in, left=1in, right=1in, bottom=1in]{geometry}  
\usepackage{graphicx}
\usepackage[usenames,dvipsnames]{xcolor}
\usepackage{scrpage2}
\usepackage{background}
\usepackage{lipsum}
\usepackage{tikz}

\colorlet{MyColor1}{RoyalPurple}
\newcommand{\HRule}{\rule{\textwidth}{1.5pt}}
\pagestyle{scrheadings}
\clearscrheadfoot
\chead{{\color{MyColor1} \textsc{\Huge{My Name} \\[-1.5pt] \Large{some tagline}}\\ \Large{\vspace{-8.5pt}} \HRule}}
\footskip = 0pt
\cfoot{\newcommand{\NL}{ $\ast$ }
{\color{MyColor1} \HRule \textsc{\footnotesize{\\ My address} \footnotesize{ \\ my.email@server.com \NL webpage.com \\ +44 123 456 789}}}}

\backgroundsetup{contents={\tikz[blend mode=screen]{\node (img){%
\includegraphics[width=0.1\textwidth]{example-image-a}};
\fill[MyColor1](img.south east) rectangle (img.north west);}},%
 angle=0, position=current page.south, anchor=above, vshift=10pt}


\begin{document}
{\color{MyColor1}\Huge Title}\\[1cm]
\lipsum[1-5]
\end{document}

Answer 2

Package xcolor provides useful macros for extracting and converting color specifications. The following example uses them to convert the color into the color model rgb. The values are parsed to construct the decodearray.

Requirement:

• The image must be a bitmap image in the RGB color space.

The image is generated with the following TeX code:

\documentclass{standalone}
\begin{document}
  \setlength{\fboxrule}{1mm}
  \fbox{\sffamily\bfseries ABC}
\end{document}

It is converted via Ghostscript and the device pngalpha to abc.png:

Now the document that includes the image:

\documentclass{article}

\usepackage{graphicx}
\usepackage[usenames,dvipsnames]{xcolor}
\colorlet{MyColor1}{RoyalPurple}

\makeatletter
\newcommand*{\ParseColorToRgbDecodeArray}[2]{%
  \extractcolorspecs{#1}\ParseResultModel\ParseResultColor
  \convertcolorspec\ParseResultModel\ParseResultColor{rgb}\ParseRgb
  \let#2\@empty
  \@for\ParseValue:=\ParseRgb\do{%
    \edef#2{%
      #2%
      \ifx#2\@empty
      \else
        \space
      \fi
      \ParseValue\space 1%
    }%
  }%
}
\makeatother

\ParseColorToRgbDecodeArray{MyColor1}\MyColorOneDecodeArray

\begin{document}
  \includegraphics[decodearray={\MyColorOneDecodeArray}]{abc.png}
\end{document}

Caveat:

The original color was given as CMYK color. The image uses RGB. Therefore, the colors in the different models can be displayed differently.