Using background colors in monospaced text

It was fun :)

You asked two questions, so there are two answers:

The first one is the box. I copied the definition of xcolor's \colorbox, modified it, and called it \tallcbox. The difference is that the size of the \colorbox is the size of its contents plus \fboxsep to each side.

I changed \tallcbox to have the same width as the contents, and have height and depth equal to the height and depth of a \strut.

The second is the "more elegant way of doing this". Elegance is subjective, so I presumed that something along the lines of "less code" and "less" is what you want :)

I defined another command, \letterbox, which take the same arguments as \colorbox or \tallcbox. But \letterbox first converts the given color to grayscale using \colorlet{temp@c}[gray]{#1}. Then it uses \extractcolorspec{temp@c}{\color@spec} to get the properties of this converted color into \color@spec. The \color@spec for a gray color will be {gray}{<0--1>}, where <0--1> denotes a number, you guesses, between 0 and 1. Finally, \letterbox compares this number with 0.5 and sets the text color to black or white, depending on the value of <0--1>.

Here's the code:

\documentclass[parskip]{scrbook}
\usepackage{xcolor}

\makeatletter
\def\black@or@white#1#2{%
  \@tempdima#2 pt
  \ifdim\@tempdima>0.5 pt
    \definecolor{temp@c}{gray}{0}%
  \else
    \definecolor{temp@c}{gray}{1}%
  \fi}
\def\letterbox#1#{\protect\letterb@x{#1}}
\def\letterb@x#1#2#3{%
  \colorlet{temp@c}[gray]{#2}%
  \extractcolorspec{temp@c}{\color@spec}%
  \expandafter\black@or@white\color@spec
  {\color#1{temp@c}\tallcbox#1{#2}{#3}}}
\def\tallcbox#1#{\protect\color@box{#1}}
\def\color@box#1#2{\color@b@x\relax{\color#1{#2}}}
\long\def\color@b@x#1#2#3%
 {\leavevmode
  \setbox\z@\hbox{{\set@color#3}}%
  \ht\z@\ht\strutbox
  \dp\z@\dp\strutbox
  {#1{#2\color@block{\wd\z@}{\ht\z@}{\dp\z@}\box\z@}}}
\makeatother

\begin{document}
    \texttt{AjuC ER7 (R) (24)%
        E%
        \letterbox{black}{VRAAGLNF}%
        L%
        \letterbox{black}{DVL}%
        S%
        \letterbox{black}{A}%
        \letterbox{gray}{M}%
        \letterbox{black}{GM}%
        \letterbox{red}{R}%
        \letterbox{black}{P}%
        DVEPGGVPR%
        \letterbox{black}{LGGECA}%
    }\\
    \texttt{AjuE ER9 (R) (24)%
        R%
        \letterbox{black}{V}%
        E%
        \letterbox{black}{AAGLNFRDVL}%
        VS%
        \letterbox{black}{LG}%
        G%
        \letterbox{red}{R}%
        ID--QS%
        \letterbox{black}{D}%
        EII%
        \letterbox{black}{LGGECA}%
    }
\end{document}

---

Ooh, and as the other answer says, you have to protect the end of the lines with % to kill the extra spaces.

Use comment at end of line % and give \setlength{\fboxsep}{0pt}

\documentclass[parskip]{scrbook}
\usepackage{xcolor}
\setlength{\fboxsep}{0pt}
\begin{document}
    \texttt{AjuC ER7 (R) (24)
        E%
        \color{white}\colorbox{black}{VRAAGLNF}%
        \color{black}L%
        \color{white}\colorbox{black}{DVL}%
        \color{black}S%
        \color{white}\colorbox{black}{A}%
        \colorbox{gray}{M}%
        \colorbox{black}{GM}%
        \colorbox{red}{R}%
        \color{white}\colorbox{black}{P}%
        \color{black}DVEPGGVPR%
        \color{white}\colorbox{black}{LGGECA}
    }\\
    \texttt{AjuE ER9 (R) (24)
        R%
        \color{white}\colorbox{black}{V}%
        \color{black}E%
        \color{white}\colorbox{black}{AAGLNFRDVL}%
        \color{black}VS%
        \color{white}\colorbox{black}{LG}%
        \color{black}G%
        \color{white}\colorbox{red}{R}%
        \color{black}ID--QS%
        \color{white}\colorbox{black}{D}%
        \color{black}EII%
        \color{white}\colorbox{black}{LGGECA}
    }
\end{document}

Here's the output

I'm almost re-taking the answer by @Mu30 murugans2katgmail and adding the vertical padding with the strut trick (here created with an invisible rule):

\documentclass[parskip]{scrbook}
\usepackage{xcolor}
\setlength{\fboxsep}{0pt}

\let\oldcolbox\colorbox
\renewcommand\colorbox[2]{%
        \oldcolbox{#1}{\rule[-.5ex]{0pt}{2.5ex}#2}%
    }%
\begin{document}
    \texttt{AjuC ER7 (R) (24) E\color{white}\colorbox{black}{VRAAGLNF}\color{black}L\color{white}\colorbox{black}{DVL}\color{black}S\color{white}\colorbox{black}{A}\colorbox{gray}{M}\colorbox{black}{GM}\colorbox{red}{R}\color{white}\colorbox{black}{P}\color{black}DVEPGGVPR\color{white}\colorbox{black}{LGGECA}
    }\\
    \texttt{AjuE ER9 (R) (24)
        R\color{white}\colorbox{black}{\strut V}\color{black}E\color{white}\colorbox{black}{AAGLNFRDVL}\color{black}VS\color{white}\colorbox{black}{LG}\color{black}G\color{white}\colorbox{red}{R}\color{black}ID--QS\color{white}\colorbox{black}{D}\color{black}EII\color{white}\colorbox{black}{LGGECA}
    }
\end{document}

You can control spacing above and below the letter by changing the value of the \rule.

EDIT: as suggested if you want exactly the vertical padding to be centered just substitute the \rule with a \strut:

\documentclass[parskip]{scrbook}
\usepackage{xcolor}
\setlength{\fboxsep}{0pt}


\let\oldcolbox\colorbox
\renewcommand\colorbox[2]{%
        \oldcolbox{#1}{\strut#2}%
    }%
\begin{document}
    \texttt{AjuC ER7 (R) (24) E\color{white}\colorbox{black}{VRAAGLNF}\color{black}L\color{white}\colorbox{black}{DVL}\color{black}S\color{white}\colorbox{black}{A}\colorbox{gray}{M}\colorbox{black}{GM}\colorbox{red}{R}\color{white}\colorbox{black}{P}\color{black}DVEPGGVPR\color{white}\colorbox{black}{LGGECA}
    }\\
    \texttt{AjuE ER9 (R) (24)
        R\color{white}\colorbox{black}{\strut V}\color{black}E\color{white}\colorbox{black}{AAGLNFRDVL}\color{black}VS\color{white}\colorbox{black}{LG}\color{black}G\color{white}\colorbox{red}{R}\color{black}ID--QS\color{white}\colorbox{black}{D}\color{black}EII\color{white}\colorbox{black}{LGGECA}
    }
\end{document}