How to use data structures in TikZ?

I don't know about expl3 but it is possible within TikZ, or rather, within pgfkeys. Since there is an l3keys submodule of LaTeX3, I assume basically the same idea works there. You can create your data structure in much the same way as in Perl:

\pgfkeys{
 /names/.cd,
 make name/.style = {
  #1/color/.initial = black
 },
 make name/.list = {Katie, Frank, Laura},
 Frank/color = blue
}

Then you can extract the color of a name via, say:

\newcommand\getcolor[1]{%
 \pgfkeysgetvalue{/names/#1/color}%
}

which is fully expandable and can be used wherever you need the color of a name. Of course, for this example there is no need for the single subkey Frank/color (you could just set Frank/.initial = blue) but if you want more properties, you can add them by giving them their own named subkeys as well.

This ultimately does not answer to the question:

Is it possible to implement this in expl3?

thus it is marked community wiki.

Besides the interesting aspect of dealing with expl3, this job is perfectly doable just with the standard syntax of TikZ foreach in combination with xstring:

\documentclass{article}
\usepackage{tikz} % won't work just with pgffor
\setlength{\parindent}{0cm}
\usepackage{xstring}
\begin{document}

\foreach \name [count=\i from 0]in{Katie,Frank=>blue,Laura=>red}{
   \IfSubStr{\name}{=>}{% true
     \StrCut{\name}{=>}\xname\namecol
     \i: \textcolor{\namecol}{\xname}\par%
   }{% false
     \i: \name\par%
   }
}

\end{document}

This provides:

The idea behind is more or less like this:

1. look for the separator string (=> in the example);
2. if present cut the string into two parts (name and color) and use them;
3. if not present, use the standard syntax.

Of course, it is possible to:

• change separator string;
• do much more complicated things (i.e. adding more properties: just nest some conditionals or, in a simpler manner, define one separator string per property).