Using \columncolor outside column definition

Question

I'm trying to create a command, which calls on an environment, which defines a custom column definition with one argument using \newcolumntype, and defines a second command, containing parameters for a table column. Then, still within the first command, I begin the tabular environment and call the custom column, with the second command as its argument. I always get an undefined control sequence error, though. Any thoughts on why that might be?

I'm sure that was clear as mud, but I've cut it down to a minimal example, which follows:

\documentclass{article}
\usepackage{array}
\usepackage[table,dvipsnames]{xcolor}
\usepackage{colortbl}

\newenvironment{tablecolor}{
    \newcommand{\tlight}{\columncolor{SkyBlue!30}\color{black}}
    \newcolumntype{C}[1]{>{##1}c}
}{}

\newcommand{\tablegoals}{
\begin{tablecolor}
\begin{tabular}{l|C{\tlight}}
Goal 7  & \\
\end{tabular}
\end{tablecolor}
}

\begin{document}
    \tablegoals
\end{document}

---

Here's my second attempt. Different approach, same undefined control sequence error with \columncolor.

\documentclass{article}
\usepackage{array}
\usepackage[table,dvipsnames]{xcolor}
\usepackage{colortbl}
\usepackage{ifthen}

\newenvironment{tablecolor}{
    \newcolumntype{C}[1]{>{
        \ifthenelse{\equal{dark}{##1}}{
            \columncolor{Cyan}\color{white}
            }{
            \columncolor{SkyBlue!30}\color{black}
        }
    }c}
}{
}

\newcommand{\tablegoals}[1]{
    \begin{tablecolor}
        \begin{tabular}{l|C{dark}}
            Goal 7  &   #1
        \end{tabular}
    \end{tablecolor}
}

\begin{document}
    \tablegoals{Check mark}
\end{document}

Answer 1

I asked David Carlisle, who wrote both the colortbl package and \newcolumntype in the array package about this problem. Here's his response:

the error message is telling you that \columncolor is not defined and in fact looking at colortbl.sty there is no definition for such a command, it is just used as a token in

\expandafter\CT@extract\the\toks\@tempcnta\columncolor!\@nil

which means essentially that after the normal array package table processing has done its stuff, colortbl goes in finds that token as a marker and inserts some extra stuff.

I can't remember why it was written that way (the log says it's 15 years ago:-) but probably there was a reason at the time.

The result (which is probably not documented too well) is that if you hide \columncolor in a macro it won't work, it has to be there explicitly at the top level. You could use newcolumntypes defining one color or the other, or taking a parameter taking the colour, as they are expanded out by array.sty before colortbl starts looking for its tokens.

Answer 2

Wow, you've got a bit of a mess on your hands there! First off, you don't need that new environment, or at least you don't need those commands in it. Try this:

\documentclass{article}
\usepackage{array}
\usepackage[table,dvipsnames]{xcolor}
\usepackage{colortbl}
\newenvironment{tablegoals}[1]
    {\begin{tabular}{l|>{\columncolor{#1}}c}}
    {\end{tabular}}

\begin{document}
    \begin{tablegoals}{SkyBlue!30}
    Goal 7  & Something \\
    \end{tablegoals}
\end{document}

The \newwnvironment here takes the name of your new environment as the first argument, the stuff that comes before the content of your environment as the second, and the stuff at the end your environment as the third. Thus the second argument contains the \begin{tabular}{...} including the colour definition of the second column. The third argument simply ends the table. There is no need to define any new commands, since this new environment allows you to type all this only once anyway.

If you find yourself defining a lot of tables with different column colours, then perhaps the following newcommand can help:

\newcommand{\newcolumnwithcolor}[2]{\newcolumntype{#1}{>{\columncolor{#2}}c}}

Now in your preamble you can define lots of column types quickly and readably

\newcolumnwithcolor{C}{SkyBlue!30}

which you may use in a tabular environment.

\begin{tabular}{l|C}
Goal 7  & Something \\
\end{tabular}

Note that is seems that these may only be named with a single character.

Finally, you can define a special column that takes arguments. If you use the same column definition and only change the colour for example, then the following works:

\newcolumntype{D}[1]{>{\columncolor{#1}}c}

\begin{tabular}{|l|D{SkyBlue!10}|}
Goal 7  & Something \\
\end{tabular}

The number of additional arguments may be more than one, so you can add alignment as an option etc.