Table border problem

Question

during my work I've encountered the following problem: I tried to prepare external border of my table. And it worked fine, to the time I applied \multicolumn command to merge two adjacent cells. The name of new merged cell is "Oznaczenia zastosowane w artykule".

Please look at screenshot below.

We can see that there is blue thin part border color at left and right from the cell "Oznaczenia zastosowane w artykule":

How to improve that and keep that color of external thicker border at both sides (left and right)?

Below is my MWE code:

\documentclass[aps,physrev,showkeys,twocolumn,nofootinbib,floatfix]{revtex4-2}
\usepackage[polish]{babel}
\selectlanguage{polish}
\usepackage{polski}
\usepackage[table,xcdraw]{xcolor}
\usepackage{array,booktabs}
\usepackage{multirow}
\usepackage{tabularx}
\usepackage{setspace}
\newcommand\VRule[1][\arrayrulewidth]{\vrule width #1}


\begin{document}

\onecolumngrid
\newpage
\arrayrulecolor{blue}
\begin{table}[htbp]

\begin{tabular}{ !{\color{red}\VRule[.5mm]} l    l !{\color{green}\VRule[.5mm]}  } \specialrule{.5mm}{2pt}{0cm}
\hfill & \hfill \\[0.25cm]
\multicolumn{2}{|l|} {\hspace{5mm}\textbf{Oznaczenia zastosowane w artykule} }  \\ [4mm]
\hspace{5mm} \textbf{Symbol} & \textbf{Opis} \\ [4.5mm]
\hspace{5mm} $\mathrm{ a }$ \hspace{3cm} & --~połowa długości [długość] pęknięcia w próbce M(T) [CT]  \\ 
\hspace{5mm} $\mathrm{ BFS }$ & --~oznaczenie pozycji czujnika \\
\hspace{5mm} $\mathrm{ CC }$ & --~zamykanie się pęknięcia  \\ \specialrule{2pt}{0pt}{0pt}
\end{tabular}
\end{table}
\twocolumngrid

\end{document}

Answer 1

you specified normal rules

 \multicolumn{2}{|l|}

I guess you wanted

\multicolumn{2}{!{\color{red}\VRule[.5mm]}l!{\color{red}\VRule[.5mm]}} 

Answer 2

You may be interested write your table by using of the tabularray package. Its way of writing of multi column cells doesn't overwrite in table preamble defined rules, so your problem cannot be happen:

\documentclass[aps,physrev,showkeys,twocolumn,nofootinbib,floatfix]{revtex4-2}
\usepackage[polish]{babel}
\selectlanguage{polish}
\usepackage{polski}
\usepackage[table,xcdraw]{xcolor}
\usepackage{tabularray}


\begin{document}

\onecolumngrid
\newpage
    \begin{table}[htbp]
\begin{tblr}{hline{1,Z} = {0.5mm,blue}, vline{1,Z} = {0.5mm, red},
             colspec  = {Q[l, mode=math, leftsep=4mm, wd=9em] Q[l]},
             row{1,2} = {font=\bfseries, mode=text},
             row{1-3} = {abovesep=4mm}
             }
\SetCell[c=2]{l}    Oznaczenia zastosowane w artykule     % multicolumn cell    
                &                                                           \\ 
Symbol          &   Opis                                                    \\ 
\mathrm{a}      & -- połowa długości [długość] pęknięcia w próbce M(T) [CT] \\
\mathrm{BFS}    & -- oznaczenie pozycji czujnika                            \\
\mathrm{CC}     & -- zamykanie się pęknięcia                                \\ 
\end{tblr}
    \end{table}
\twocolumngrid

\end{document}

In comparison with your MWE, proposed solution ha the following differences:

• first column has left column separation equal 4mm, so you not need to insert horizontal spaces to their cell
• first column is in math node (consequently you not need to write $)
• First three row has above vertical space defined in table preamble, so [5.5mm]˛ after rows terminators are removed
• first two rows are in text mode with `bfseries font