rowcolor for a multirow

I propose the following improvement to the table: as booktabs introduces vertical spacing around rules, which results in white strips in coloured rows, I replace the rule commands from booktabs with variable width rule commands from the small boldline package, from the shipunov bundle, and add vertical padding above and below row cells with cellspace package, which ensures a minimal vertical spacing at the top and the bottom of every cell of columns prefixed by the letter S.

documentclass{article}

usepackage{caption}
usepackage[table]{xcolor}
usepackage{multirow}
usepackage{boldline}
usepackage{cellspace}
setlengthcellspacetoplimit{4pt}
setlengthcellspacebottomlimit{4pt}

begin{document}

begin{table}[h]
  caption{caption}
  label{tab}
  centering
  begin{tabular}{*{6}{Sc}}
    hlineB{2}
    & & A & B & C & D 
   hlineB{1.25}
    rowcolor{gray!10}
    & Pruning & -- & -- & 6-C-Pr & -- 
    rowcolor{gray!10}
    multirow{-2}{*}{Rosso}
    & Stalks & -- & -- & 6-C-S & -- 
    & Pruning & -- & -- & 6-M-Pr & -- 
    multirow{-2}{*}{Verde}
    & Stalks & -- & -- & 6-M-S & -- 
    rowcolor{gray!10}
    & Pruning & 4-Pa-Pr & 5-Pa-Pr & 6-Pa-Pr & 7-Pa-Pr 
    rowcolor{gray!10}
    multirow{-2}{*}{Blu}
    & Stalks & 4-Pa-S & 5-Pa-S & 6-Pa-S & 7-Pa-S 
    & Pruning & -- & -- & 6-W-Pr & -- 
    multirow{-2}{*}{Bianco}
    & Stalks & -- & -- & 6-W-Pr & -- 
    hlineB{2}
  end{tabular}
end {table}

end{document} 

You can construct that table with {NiceTabular} of nicematrix with the following avantages:

• The colored rows will automatically respect the rules of booktabs.
• You don't have to put multicolumn with a negative argument: you use Block with a positive argument.
• You can color alternatively the logical rows with a command rowcolors in the CodeBefore.
• You will have a good output in all the PDF viewers at all levels of zoom (better than with colortbl).

There is a drawback: you need several compilations (because nicematrix uses PGF/Tikz nodes under the hood).

documentclass{article}
usepackage{caption}
usepackage{nicematrix}
usepackage{booktabs}

begin{document}


begin{table}[h]
caption{caption}
label{tab}
centering
begin{NiceTabular}{*{6}{c}}[cell-space-limits=4pt]
CodeBefore
  rowcolors{1}{}{gray!10}[respect-blocks]
Body
  toprule
  & & A & B & C & D 
 midrule
  Block{2-1}{Rosso}
  & Pruning & -- & -- & 6-C-Pr & -- 
  & Stalks & -- & -- & 6-C-S & -- 
  Block{2-1}{Verde}
  & Pruning & -- & -- & 6-M-Pr & -- 
  & Stalks & -- & -- & 6-M-S & -- 
  Block{2-1}{Blu}
  & Pruning & 4-Pa-Pr & 5-Pa-Pr & 6-Pa-Pr & 7-Pa-Pr 
  & Stalks & 4-Pa-S & 5-Pa-S & 6-Pa-S & 7-Pa-S 
  Block{2-1}{Bianco}
  & Pruning & -- & -- & 6-W-Pr & -- 
  & Stalks & -- & -- & 6-W-Pr & -- 
  bottomrule
end{NiceTabular}
end {table}

end{document}