Highlighting text through stacked colored underlines

Question

I have sentences in which characters have been "marked" by some people and not by others. I'd like to present the markings simultaneously. Stacked colored underlining (in which a line represents each person's marks) seems like the best solution. However, I'm struggling to make that happen. How can I present multiple markings simultaneously?

Goal

Code That Fails

Because } closes the most recent {, the following code only works for a subset of examples (it fails on the goal example):

\documentclass{article}
\usepackage{xcolor}
\newcommand{\rul}[1]{\textcolor{red}{\underline{\textcolor{black}{#1}}}}
\newcommand{\bul}[1]{\textcolor{blue}{\underline{\textcolor{black}{#1}}}}

\begin{document}
\rul{sample \bul{with blue embedded} and continuing red}
\end{document}

I've experimented a bit with \lefteqn and \phantom. Using these commands resolves the bracket mismatching, but it has two other problems: it's text in math mode, and there isn't a vertical gap between the two underlining colors:

\[\lefteqn{\rul{\phantom{overlapping high}}}overlapping \bul{highlighting}\]

Other Considerations

I need to make many pages of this sort of text. For readability, the solution should have the following properties:

• Underlining automatically wraps across lines.
• Underlining of a single color stays on the same horizontal level throughout the document (e.g., blue is always n units below baseline).
• Scales up to ~10 stacked underlines in a document section.

Because of the amount of this text, a readable/maintainable solution would be aces.

Answer 1

This answer will NOT wrap. Nonethless, proceeding...

Here, I introduce \nunderline[]{}{}. The optional argument is the under-level for rule placement (relative to the prior placement). The first argument is the text, and the second argument is the color. The rule thickness is set with \rulethick and th relative spacing with \lunderset.

Nesting is used to obtain multiple lines under a given phrase. To take you through it, line by line:

1. underline "overlap" in red%

2. underline "ping " in red and then, 2 levels below that, in cyan%

3. underline "high" in red, then blue, then cyan%

4. underline "light" at the 2nd level in blue, and below that, in cyan%

5. underline "ing" at the 3rd level in cyan.

\documentclass{article}
\usepackage{stackengine}
\usepackage{xcolor}
\newlength\lunderset
\newlength\rulethick
\lunderset=1.5pt\relax
\rulethick=.8pt\relax
\def\stackalignment{l}
\newcommand\nunderline[3][1]{\setbox0=\hbox{#2}%
  \stackunder[#1\lunderset-\rulethick]{\strut#2}{\color{#3}\rule{\wd0}{\rulethick}}}
\begin{document}
\nunderline{overlap}{red}%
\nunderline[2]{\nunderline{ping }{red}}{cyan}%
\nunderline{\nunderline{\nunderline{high}{red}}{blue}}{cyan}%
\nunderline{\nunderline[2]{light}{blue}}{cyan}%
\nunderline[3]{ing}{cyan}
\end{document}

Answer 2

Here's a completely different approach that DOES allow line-wrapping, but not paragraph or page breaks. It is also forced to turn off hyphenation. It uses the censor package to create the underlining (by setting \censorruleheight and \censorruledepth), and it uses a \Longunderstack to stack the different threads at the proper spacing (based on result at Possible to wrap multiple rows within a column at one time?).

The first \thread is the unadulterated text, whereas subsequent threads in the stack (note the index on the first argument) use \whiteout and \blackout to denote text to be left alone and text to be underlined with the color associated with that thread. The downside is that the complete text must appear once for each thread in your input file, which can make for large inputs.

If too many threads cause lines to run together, \baselinemag may be increased from its current 1.2 setting. I've also set the thread width to 3in, which can be changed.

Note: \blackout and \whiteout cannot end its argument on a space.

\documentclass{article}
\usepackage[none]{hyphenat}
\usepackage{censor}
\censorruleheight=.7pt
\usepackage{xcolor}
\usepackage{stackengine}
\setstackgap{L}{0pt}
\newlength\ruleskip\newlength\rulebase
%%YOU CAN PLAY WITH THESE
\setlength\rulebase{-1.9ex}
\setlength\ruleskip{-.3ex}
\def\baselinemag{1.2}
\def\threadwidth{3in}
%%%%%%%%%%
\def\stacktype{L}
\def\stackalignment{l}
\newcommand\thread[3]{\parbox[t]{\threadwidth}{\baselineskip=\baselinemag\baselineskip%
  \setlength\censorruledepth{\rulebase+#1\ruleskip}\color{#2}#3}}
\def\whiteout#1{\textcolor{white}{\blackout{#1}}}
\begin{document}
\raisebox{7pt}{\Longunderstack{%
\thread{0}{black}{The censor package allows a convenient redaction/censor capability to be 
employed, for those who need to protect restricted information, as well as for those
who are forced to work in a more inefficient environment when dealing with
restricted information.
}\\
\thread{1}{red}{%
  \whiteout{%
    The censor package allows a convenient redaction/censor capability to be 
    employed, for those who need to protect}%
  \blackout{ restricted information,}%
  \whiteout{ as well as for those
    who are forced to work in a more inefficient environment when dealing with
    restricted information.}}\\
\thread{2}{blue}{%
  \whiteout{%
    The censor package allows a convenient redaction/censor capability to be 
    employed, for those who need to protect restricted}%
  \blackout{ information, as well as for those
    who are forced}%
  \whiteout{ to work in a more inefficient environment when dealing with
    restricted information.}}\\
\thread{3}{cyan}{%
  \whiteout{%
    The censor package allows a convenient redaction/censor capability to be 
    employed, for those who need to pro}%
  \blackout{tect restricted inform}%
  \whiteout{ation, as well as for those
    who are forced to work in a more inefficient environment when dealing with
    restricted information.}}
}}
\end{document}