TikZ - foreach, expansion problem, optional argument and node

The problem comes indeed from the use of the name color. The use of col makes all things work... :-)

I don't know why you define a function to change nodes when all you have to do is create a style.

Here, I created the my note style which has 3 arguments : color, name, and text.

my note/.style n args={3}{text=#1,name=#2,label={[#1]right:#3}},
my note/.default={black}{}{},

documentclass[12pt]{article}

RequirePackage{forest}
usetikzlibrary{calc}
useforestlibrary{linguistics}

%newcommandptreeComment[3][black]{
%    node [anchor=mid west] at (#2.mid -| ptreecomment coord) {%
%        textcolor{#1}{#3}%
%    };
%}

begin{document}

begin{forest}
my note/.style n args={3}{text=#1,name=#2,label={[#1]right:#3}},
my note/.default={black}{}{},
    for tree = {%
        sn edges,
        grow'  = 0,
        l      = 2.5cm,
        s sep  = 1.2cm,
        anchor = parent,
    },
    tikz+={
        coordinate (ptreecomment coord) at (current bounding box.east);
    },
    [
        [$A$
            [$B$,my note]
            [$C$,my note ={red}{nC}{$X=3$}]
        ]
        [$D$
            [$E$,my note ={blue}{nE}{$X=3$}]
            [$F$,my note ={black!60!green}{nF}{$X=5$}]
        ]
    ]
    %
%     ptreeComment{nB}{$X = 1$}
%     ptreeComment[red]{nC}{$X = 3$}
%     ptreeComment[orange]{nE}{$X = 3$}
%     ptreeComment[black!60!green]{nF}{$X = 5$}
end{forest}

end{document}

You should be more careful when choosing names for local variables in foreach:

% xcolor.sty, line 756:
deftextcolor#1#{@textcolor{#1}}

% xcolor.sty, line 757:
def@textcolor#1#2#3{protectleavevmode{color#1{#2}#3}}

You're using textcolor, which internally uses color: if you redefine color you can't expect that `textcolor works; actually you clearly see why the color name is printed.

There is a different way to cope with the problem, without using local variables.

documentclass[12pt]{article}

usepackage{forest}
usetikzlibrary{calc}
useforestlibrary{linguistics}

usepackage{xparse}
ExplSyntaxOn

NewDocumentCommand{lforeach}{ s O{} m +m }
 {
  IfBooleanTF{#1}
   {
    manual_lforeach:non { #2 } { #3 } { #4 }
   }
   {
    manual_lforeach:nnn { #2 } { #3 } { #4 }
   }
 }

int_new:N g__manual_foreach_map_int

cs_new_protected:Nn manual_lforeach:nnn
 {
  keys_set:nn { manual/lforeach } { single }
  keys_set:nn { manual/lforeach } { #1 }
  clist_set:Nn l__manual_lforeach_list_clist { #2 }
  int_gincr:N g__manual_foreach_map_int
  __manual_lforeach_define:n { #3 }
  clist_map_inline:Nn l__manual_lforeach_list_clist
   {
    use:c { __manual_lforeach_ int_use:N g__manual_foreach_map_int _action:w } ##1 q_stop
   }
  int_gdecr:N g__manual_foreach_map_int
 }
cs_generate_variant:Nn manual_lforeach:nnn { no }

cs_new_protected:Nn __manual_lforeach_define:n
 {
  exp_last_unbraced:NcV
   cs_set:Npn
   { __manual_lforeach_ int_use:N g__manual_foreach_map_int _action:w }
   l__manual_lforeach_format_tl
   q_stop
   {#1}
 }

keys_define:nn { manual/lforeach }
 {
  format .tl_set:N = l__manual_lforeach_format_tl,
  single .code:n = tl_set:Nn l__manual_lforeach_format_tl { ##1 },
  double .code:n = tl_set:Nn l__manual_lforeach_format_tl { ##1/##2 },
  triple .code:n = tl_set:Nn l__manual_lforeach_format_tl { ##1/##2/##3 },
 }
ExplSyntaxOff

newcommandptreeComment[3][black]{%
    node [anchor=mid west] at (#2.mid -| ptreecomment coord) {%
        textcolor{#1}{#3}%
    };
}

begin{document}

begin{forest}
    for tree = {%
        sn edges,
        grow'  = 0,
        l      = 2.5cm,
        s sep  = 1.2cm,
        anchor = parent,
    },
    tikz+={
        coordinate (ptreecomment coord) at (current bounding box.east);
    },
    [
        [$A$
            [$B$, name = nB]
            [$C$, name = nC]
        ]
        [$D$
            [$E$, name = nE]
            [$F$, name = nF]
        ]
    ]
    % 
    lforeach[triple]% triple means #1/#2/#3
     {
      black/nB/$X = 1$,
      red/nC/$X = 3$,
      orange/nE/$X = 3$,
      black!60!green/nF/$X = 5$
     }
     {ptreeComment[#1]{#2}{#3}}
end{forest}

end{document}

The macro lforeach has an optional argument for stating options; the possible ones are

• single (default), when you only want to cycle over a standard comma separated list;
• double if the list is of the form 1/a,2/b,... and the items before and after the / are referred to with #1 and #2;
• triple if the list is of the form `1/a/i,2/b/ii,... (used in the example);
• format to choose whatever delimiters you wish, with any number of arguments (up to nine, of course); for instance, triple is the same as format=#1/#2/#3.

The *-variant expands once the first mandatory argument (if the list is stored in a macro). The first mandatory argument has the list, the second one has the code to be executed, where instead of “variables” you use #1, #2 and so on.

For instance, you get the same output with

    lforeach[format=C(#1)N(#2)T(#3)]
     {
      C(black)N(nB)T($X = 1$),
      C(red)N(nC)T($X = 3$),
      C(orange)N(nE)T($X = 3$),
      C(black!60!green)N(nF)T($X = 5$)
     }
     {ptreeComment[#1]{#2}{#3}}

(this is just for showing the possibilities).