Colored “sum” in display math mode

Question

The line

\[ \textcolor{green}{\sum}_{k=0}^n \]

makes the k=0 and n parts go to the right of Sigma instead of at the top and bottom, as it should be. Can we retain k=0 and n at the top and bottom?

Answer 1

The \textcolor macro will undo the operator status of \sum so that both subscript and superscript are placed like they would at an ordinary symbol.

The macro \mathop makes the whole sequence (the green sum symbol) an operator again.

When you often use colored operator symbols, define a new macro, say

\newcommand*{\opcolor}[2]{\mathop{\textcolor{#1}{#2}}}

and use it as \opcolor{green}{\sum}.

I have also defined a \csum macro based on \opcolor.

Code

\documentclass{article}
\usepackage{xcolor}
\newcommand*{\opcolor}[2]{\mathop{\textcolor{#1}{#2}}}
\newcommand*{\csum}[1]{\opcolor{#1}\sum}

\begin{document}
\[ \mathop{\textcolor{green}{\sum}}_{k=0}^n \]

\[ \csum{green}_{k=0}^n \]

\[ \opcolor{green}{\sum}_{k=0}^n \]
\end{document}

Output

Answer 2

Declare the green symbol as an operator via \mathop.

\documentclass{article}
\usepackage{amsmath}
\usepackage{xcolor}
\begin{document}
\[ \mathop{\textcolor{green}{\sum}}_{k=0}^n \]
\end{document}