How to remove extra black line coming in table due to hhline

Question

Using the code given below, I am getting a black line over the word "Methods" due to hhline. How shall I remove this black line or make it of same color as the background color so that it is not visible?

\documentclass{article}
  \usepackage{hhline}
 \usepackage[table]{xcolor}
   \usepackage{array}
 \usepackage{hhline}
  \usepackage{multirow}
  \definecolor{babyblue}{rgb}{0.54, 0.81, 0.94}
   \definecolor{aliceblue}{rgb}{0.94, 0.97, 1.0}
    \begin{document}
    \begin{table}[]
    \centering
    \setlength\arrayrulewidth{1.3pt}
        \footnotesize\setlength{\tabcolsep}{2.7 pt}
    \renewcommand{\arraystretch}{1.4}
    \begin{tabular}{|c|c|c|c|c|c|c|}
        \hline
        \cellcolor{aliceblue}  & \multicolumn{2}{c|}{\cellcolor{babyblue}ISE} & \multicolumn{2}{c|}{\cellcolor{babyblue}IAE} & \multicolumn{2}{c|}{\cellcolor{babyblue}ITAE} \\ \hhline{-|-|-|-|-|-|-|} \cline{2-7} 
            \multirow{-2}{*}{\cellcolor{aliceblue} Methods}
        & \cellcolor{aliceblue}-20\%       & \cellcolor{aliceblue}+20\%     & \cellcolor{aliceblue}-20\%       & \cellcolor{aliceblue}+20\%      & \cellcolor{aliceblue}-20\%       & \cellcolor{aliceblue}+20\%       \\ \hhline{-|*6-}
        \rowcolor{babyblue}ABC                                                           & 0.5645      & 0.63     & 1.8       & 1.8      & 6.1       & 6.6       \\ \hhline{-|*6-}
        \rowcolor{aliceblue}XYZ                                                            & 2.1       & 2.3      & 0.881       & 9.82      & 9.2       & 28.9       \\ \hhline{-|*6-}
        \rowcolor{babyblue}MNO                                                                & 0.68      & 0.97     & 1.62       & 1.61      & 8.9       & 8.2       \\ \hhline{-|*6-}
        \rowcolor{aliceblue} PQR                                                                 & 0.65       & 0.753     & 2.064       & 2.231      & 28.04       & 28.56       \\ \hhline{-|*6-}
    \end{tabular}
    \end{table}
   \end{document}

Answer 1

You need to change your used command \hhline{-|-|-|-|-|-|-|} to \hhline{|~|-|-|-|-|-|-|} (see the ~ at the first position marking there should be no line).

So the following MWE

\documentclass{article}

\usepackage{hhline}
\usepackage[table]{xcolor}
\usepackage{array}
\usepackage{multirow}

\definecolor{babyblue}{rgb}{0.54, 0.81, 0.94}
\definecolor{aliceblue}{rgb}{0.94, 0.97, 1.0}


\begin{document}

\begin{table}[]
  \centering
  \setlength\arrayrulewidth{1.3pt}
  \footnotesize\setlength{\tabcolsep}{2.7 pt}
  \renewcommand{\arraystretch}{1.4}
  \begin{tabular}{|c|c|c|c|c|c|c|}
    \hline
    \cellcolor{aliceblue}  & \multicolumn{2}{c|}{\cellcolor{babyblue}ISE} & \multicolumn{2}{c|}{\cellcolor{babyblue}IAE} & \multicolumn{2}{c|}{\cellcolor{babyblue}ITAE} \\ 
   \hhline{|~|-|-|-|-|-|-|} % <==========================================
    \multirow{-2}{*}{\cellcolor{aliceblue}Methods}
        & \cellcolor{aliceblue}-20\%       & \cellcolor{aliceblue}+20\%     & \cellcolor{aliceblue}-20\%        & \cellcolor{aliceblue}+20\%      & \cellcolor{aliceblue}-20\%       & \cellcolor{aliceblue}+20\%        \\ \hhline{-|*6-}
    \rowcolor{babyblue}ABC                                                           & 0.5645      & 0.63     & 1.8       & 1.8      & 6.1       & 6.6       \\ \hhline{-|*6-}
    \rowcolor{aliceblue}XYZ                                                            & 2.1       & 2.3      & 0.881       & 9.82      & 9.2       & 28.9       \\ \hhline{-|*6-}
    \rowcolor{babyblue}MNO                                                                & 0.68      & 0.97     & 1.62       & 1.61      & 8.9       & 8.2       \\ \hhline{-|*6-}
    \rowcolor{aliceblue} PQR                                                                 & 0.65       & 0.753     & 2.064       & 2.231      & 28.04       & 28.56       \\ \hhline{-|*6-}
  \end{tabular}
\end{table}
\end{document}

results in the following table:

Because you do not want to get rid of the lines I deleted the second table.

To get rid of the underlaying white line under text "Method" you need to change the command \hhline{|~|-|-|-|-|-|-|} to

\hhline{|>{\arrayrulecolor{aliceblue}}->{\arrayrulecolor{black}}|-|-|-|-|-|-|}

Please see that this code prints the first part of line (marked with - again; ~ adds the white line!) and colored it in the same color the background has.

With the following MWE

\documentclass{article}

\usepackage{hhline}
\usepackage[table]{xcolor}
\usepackage{array}
\usepackage{multirow}
\usepackage{booktabs}

\definecolor{babyblue}{rgb}{0.54, 0.81, 0.94}
\definecolor{aliceblue}{rgb}{0.94, 0.97, 1.0}


\begin{document}

\begin{table}[ht]
  \centering
  \setlength\arrayrulewidth{1.3pt}
  \footnotesize\setlength{\tabcolsep}{2.7 pt}
  \renewcommand{\arraystretch}{1.4}
  \begin{tabular}{|c|c|c|c|c|c|c|}
    \hline
    \cellcolor{aliceblue}  & \multicolumn{2}{c|}{\cellcolor{babyblue}ISE} & \multicolumn{2}{c|}{\cellcolor{babyblue}IAE} & \multicolumn{2}{c|}{\cellcolor{babyblue}ITAE} \\ 
   \hhline{|>{\arrayrulecolor{aliceblue}}->{\arrayrulecolor{black}}|-|-|-|-|-|-|} % <==========================================
    \multirow{-2}{*}{\cellcolor{aliceblue}Methods}
        & \cellcolor{aliceblue}-20\%       & \cellcolor{aliceblue}+20\%     & \cellcolor{aliceblue}-20\%        & \cellcolor{aliceblue}+20\%      & \cellcolor{aliceblue}-20\%       & \cellcolor{aliceblue}+20\%        \\ \hhline{-|*6-}
    \rowcolor{babyblue}ABC                                                           & 0.5645      & 0.63     & 1.8       & 1.8      & 6.1       & 6.6       \\ \hhline{-|*6-}
    \rowcolor{aliceblue}XYZ                                                            & 2.1       & 2.3      & 0.881       & 9.82      & 9.2       & 28.9       \\ \hhline{-|*6-}
    \rowcolor{babyblue}MNO                                                                & 0.68      & 0.97     & 1.62       & 1.61      & 8.9       & 8.2       \\ \hhline{-|*6-}
    \rowcolor{aliceblue} PQR                                                                 & 0.65       & 0.753     & 2.064       & 2.231      & 28.04       & 28.56       \\ \hhline{-|*6-}
  \end{tabular}
\end{table}

\end{document}

you get the result:

Answer 2

i would redesign your table as follows:

\documentclass{article}
\usepackage{siunitx}
\usepackage[table]{xcolor}
\definecolor{babyblue}{rgb}{0.54, 0.81, 0.94}
\definecolor{aliceblue}{rgb}{0.94, 0.97, 1.0}
\usepackage{booktabs, multirow}
\newcommand\mcc[1]{\multicolumn{2}{c}{#1}}

\begin{document}

\begin{table}[htb]
  \centering
  \renewcommand\arraystretch{1.2}
  \rowcolors{2}{white}{aliceblue}
  \begin{tabular}{c *{6}{S} }
    \toprule
        & \mcc{ISE} & \mcc{IAE} & \mcc{ITAE}                \\
    \cmidrule(lr){2-3}\cmidrule(lr){4-5}\cmidrule(lr){6-7}
\multirow{-2.3}{*}{Methods}
        & {$-20\%$} & {$+20\%$}
                    & {$-20\%$} & {$+20\%$}
                                & {$$-20\%$} & {$+20\%$}         \\
    \hline
ABC     & 0.5645    & 0.63  & 1.8   & 1.8   & 6.1   &  6.6  \\
XYZ     & 2.1       & 2.3   & 0.881 & 9.82  & 9.2   & 28.9  \\
MNO     & 0.68      & 0.97  & 1.62  & 1.61  & 8.9   &  8.2  \\
PQR     & 0.65      & 0.753 & 2.064 & 2.231 & 28.04 & 28.56 \\
    \bottomrule
  \end{tabular}
\end{table}
\end{document}

Answer 3

Here is a solution for the white line in the \multirow. intead of using \hhline{~|-|-|-|-|-|-|}, I use a full width \hhline , but make the first segment coloured with the background colour of the \multirow.

I also took the liberty to simplify your code, and use the AliceBlue colour from svgnames, as it doesn't seem different from your aliceblue.

\documentclass{article}

\usepackage{hhline}
\usepackage[table, svgnames]{xcolor}
\usepackage{array}
\usepackage{multirow}
\usepackage{booktabs}

\definecolor{babyblue}{rgb}{0.54, 0.81, 0.94}

\begin{document}

\begin{table}[ht]
  \centering
  \setlength\arrayrulewidth{1.3pt}
  \footnotesize\setlength{\tabcolsep}{2.7 pt}
  \renewcommand{\arraystretch}{1.4}
  \begin{tabular}{|c|c|c|c|c|c|c|}
    \hline
    \cellcolor{aliceblue} & \multicolumn{2}{c|}{\cellcolor{babyblue}ISE} & \multicolumn{2}{c|}{\cellcolor{babyblue}IAE} & \multicolumn{2}{c|}{\cellcolor{babyblue}ITAE} \\
   \hhline{|>{\arrayrulecolor{AliceBlue}}->{\arrayrulecolor{black}}|*6{-}} % <==========================================
\rowcolor{AliceBlue} \multirow{-2}{*}{Methods}
        &$ - $20\% & +20\% & $ - $20\% & +20\% & $ - $20\% & +20\% \\ \hhline{-|*6-}
    \rowcolor{babyblue}ABC & 0.5645 & 0.63 & 1.8 & 1.8 & 6.1 & 6.6 \\ \hhline{-|*6-}
    \rowcolor{AliceBlue}XYZ & 2.1 & 2.3 & 0.881 & 9.82 & 9.2 & 28.9 \\ \hhline{-|*6-}
    \rowcolor{babyblue}MNO & 0.68 & 0.97 & 1.62 & 1.61 & 8.9 & 8.2 \\ \hhline{-|*6-}
    \rowcolor{AliceBlue} PQR & 0.65 & 0.753 & 2.064 & 2.231 & 28.04 & 28.56 \\ \hhline{-|*6-}
  \end{tabular}
\end{table}

\end{document}