Error using a defined function with expl3 inside the command \color

Question

In the following MWE my personal definition of \marco_prop_get:nn works unless it isn't inside the macro \color. The snippet is:

\ExplSyntaxOn
\cs_new_protected:Npn \marco_prop_get:nn #1 #2
 {
  \prop_get:cn  { l_marco_ #1 _collection_prop } { #2 }
 }
\color{ \marco_prop_get:nn { color } {my-title} } My~ Title
\ExplSyntaxOff

I am a little bit confused because the using of \prop_get:Nn works well.

\ExplSyntaxOn
\prop_new:N \l_marco_color_collection_prop
\prop_put:Nnn \l_marco_color_collection_prop {my-title} {red}
\color{ \prop_get:Nn\l_marco_color_collection_prop {my-title} } My~ Title
\ExplSyntaxOff

Here the MWE:

\documentclass{article}
\usepackage{xcolor}
\usepackage{expl3}

\begin{document}
\def\foo{red}
\color{\foo}\foo

\ExplSyntaxOn
\cs_set:Npn \foo { blue }
\ExplSyntaxOff
\color{\foo}\foo

\ExplSyntaxOn
\prop_new:N \l_marco_color_collection_prop
\prop_put:Nnn \l_marco_color_collection_prop {my-title} {red}
\color{ \prop_get:Nn\l_marco_color_collection_prop {my-title} } My~ Title
\ExplSyntaxOff

\ExplSyntaxOn
\cs_new_protected:Npn \marco_prop_get:nn #1 #2
 {
  \prop_get:cn  { l_marco_#1_collection_prop } { #2 }
 }
\exp_after:wN \color \exp_after:wN { \marco_prop_get:nn { color } {my-title} } My~ Title

%\color {  \marco_prop_get:nn { color } {my-title} } My~ Title
\ExplSyntaxOff
\end{document}

Can you explain please why does my command fail without \exp_after:wN.

Please note I am using the svn version of expl3.

Answer 1

It would work with color but as far as I can see xcolor uses \@onelevel@sanitize to suppress expansion so the argument is safe for it to parse its more extensive color syntax. So you need to make sure you do enough expansion before \@onelevel@sanitize uses \meaning. Hence the need for \expandafter.