Spacing for subscripts in mathmode with \textcolor

Question

It is possible to use \textcolor in math-mode; however, it changes the spacing of subscripts.

\documentclass{article}
\usepackage[usenames]{xcolor}
\begin{document}
$Y_i$ vs $\textcolor{red}{Y}_i$ vs $\mathord{\textcolor{red}{Y}}_i$ vs $\color{red}Y\color{black}_i$ vs $\begingroup\color{red}Y\endgroup_i$
\end{document}

Is there any way of fixing the spacing? This suggestion (the last two examples) also didn't help.

An additional complication is that I intend to use this within a macro. So, I am aiming for something like:

\newcommand{\parameter}{\textcolor{red}{Y}}
We use both $\parameter$ and $\parameter_i$

tex --version outputs TeX 3.1415926 (TeX Live 2013).

Answer 1

With \textcolor{red}{Y}_{i} you lose the font metric information about the “Y”, so TeX can't kern the subscript. Here's a macro that works in the easiest cases (no superscript and just at the main level (not in subscripts or superscripts). If you need the full set of features, it would be just more complicated.

\documentclass{article}
\usepackage{xcolor}

\makeatletter
\newcommand\parameter{%
  \textcolor{red}{Y}%
  \@ifnextchar_{\parameter@subscript}{}%
}
\newcommand\parameter@subscript[2]{%
  % #1 is _, #2 is the actual subscript
  \sbox0{$Y_{#2}$}\sbox2{$Y{}_{#2}$}%
  _{\kern\dimexpr\wd0-\wd2\relax#2}%
}
\makeatother

\begin{document}
$Y_i$

$\parameter_i$
\end{document}

An easier implementation that works also in subscripts.

\documentclass{article}
\usepackage{xcolor}

\makeatletter
\newcommand\parameter{%
  \begingroup
  \color{red}Y\@ifnextchar_{\do@parameter}{\endgroup}%
}
\newcommand{\do@parameter}[2]{%
  _{\color{black}#2}\endgroup
}
\makeatother
\begin{document}
$Y_i$

$\parameter_i$

$A_{\parameter_i}$
\end{document}

Answer 2

May be you need this:

\def\parameter{\color{red}Y\futurelet\next\parameterA}
\def\parameterA{\ifx\next_\expandafter\parameterB\else\color{black}\fi}
\def\parameterB_#1{_{\color{black}#1}\color{black}}

$Y_i, \parameter_i, \parameter$ etc.

Edit: The code above was my first attempt to solve this problem. But two thinks were not treated here. First: color stack (the outer color may be blue, for example) and second: exponent (you can write \parameter_i^2 for example). This is a reason why I suggest a second version of my macros:

\def\parameter{\bgroup\colorlet{outcolor}{.}\color{red}Y\futurelet\next\parameterA}
\def\parameterA{\ifx\next_\expandafter\parameterB\else\egroup\fi}
\def\parameterB_#1{_{{\color{outcolor}#1}}\futurelet\next\parameterC}
\def\parameterC{\ifx\next^\expandafter\parameterD\else\egroup\fi}
\def\parameterD^#1{^{\color{outcolor}#1}\egroup}

The explanation of the problem: The index is positioned without italic correction of base character but exponent with italic correction. The previous sentence is not true if the last object of the base isn't character, i.e it is \pdfliteral or \special in order to return to the outer color. Then the italic correction is applied before such \pdfliteral. The following index (if exists) cannot remove the space from this italic correction after that. The result is: we cannot close the color in the base. We need to return to the outer color in the index manually.

Second problem: we cannot close the group (or to place \pdfliteral) after the index because the exponent (if exists) is positioned after such closing group and it is too far from the base. This is a reason why the exponent is treated similar way in may macros and the \egroup is set after exponent. Watch to the last trick: the double braces {{..}} in the index. The color setting in the xcolor LaTeX package is implemented as TeX-groups dependend by \aftergroup primitive. This places some material after the group where \color command is used. We need to save this material inside the index, no after the index is closed.

Answer 3

I combined @egreg and @wipet's answers as my final solution, which seems to work with all test cases I have tried. It has the added feature of allowing the text to be set through a parameter, so it operates analogously to \textcolor.

\documentclass{article}
\usepackage[usenames]{xcolor}

\makeatletter
\newcommand{\mathcolor}[2]{%
  \begingroup
  \colorlet{out}{.}\color{#1}#2\@ifnextchar_{\do@mathcolorsub}{\endgroup}%
}
\newcommand{\do@mathcolorsub}[2]{%
  _{{\color{out}#2}}\@ifnextchar^{\do@mathcolorsup}{\endgroup}%
}
\newcommand{\do@mathcolorsup}[2]{%
  ^{\color{out}#2}\endgroup
}
\makeatother

It makes it easy to define multiple parameters.

\newcommand{\Y}{\mathcolor{red}{Y}}
\newcommand{\A}{\mathcolor{green}{A}}
\newcommand{\Ar}{\mathcolor{blue}{\mathrm{A}}}
\newcommand{\Oc}{\mathcolor{red}{O}}

And the test cases:

\begin{document}
$Y_i+Y^i+Y_i^i+Y+Z_{Y_i}+Z^{Y_i}$\par
$\color{blue} \Y_i+\Y^i+\Y_i^i+\Y+Z_{\Y_i}+Z^{\Y_i}$\par
$A_i+A^i+A_i^i+A+Z_{A_i}+Z^{A_i}$\par
$\A_i+\A^i+\A_i^i+\A+Z_{\A_i}+Z^{\A_i}$\par
$\mathrm{A}_i+\mathrm{A}^i+\mathrm{A}_i^i+\mathrm{A}+Z_{\mathrm{A}_i}+Z^{\mathrm{A}_i}$\par
$\Ar_i+\Ar^i+\Ar_i^i+\Ar+Z_{\Ar_i}+Z^{\Ar_i}$\par
$A_i^{Y_k}+\A_i^{\Y_k}+O^i+\Oc^i$
\end{document}

I presume this is worth posting, but am happy to delete if is considered superfluous.