Lighter midrule

You can use \arrayrulecolor{<colour>} from colortbl to change the colour, and \specialrule{<width>}{<above>}{<below>} to set a rule of specific dimensions:

\documentclass{article}

\usepackage[table]{xcolor}
\usepackage{booktabs}

\begin{document}

\begin{tabular}{ c c c }
  \toprule
  A   &   B   \\
  \midrule
  Q   &   M   \\
  W   &   N   \\
  E   &   B   \\
  R   &   V   \\
  T   &   C   \\
  \arrayrulecolor{black!30}\midrule
  Y   &   X   \\
  U   &   L   \\
  I   &   K   \\
  O   &   J   \\
  P   &   H   \\
 \arrayrulecolor{black!50}\specialrule{3pt}{2\jot}{1pc}
  A   &   5   \\
  S   &   7   \\
  D   &   8   \\
  F   &   2   \\
  G   &   4   \\
  \arrayrulecolor{black}\bottomrule
\end{tabular}

\end{document}

Note that the rule colour change is global, so you'll have to reset to black if it's been changed before.

For colours you have to use \arrayrulecolor{…} at the beginning of a row. For the thickness of \midrules, you can change the value of \lightrulewidth or change the thickness of a specific rule with the optional argument. Demo:

\documentclass{article}

\usepackage{booktabs}
\usepackage[x11names, table]{xcolor}
\setlength\lightrulewidth{0.3pt}
\begin{document}

\begin{tabular}{ccc}
\toprule
A & B \\
\arrayrulecolor{Grey0!60!RoyalBlue3}\midrule[0.5pt]
Q & M \\
W & N \\
E & B \\
R & V \\
T & C \\
\arrayrulecolor{OrangeRed1! 50}\midrule
Y & X \\
U & L \\
I & K \\
O & J \\
P & H \\
\midrule
A & 5 \\
S & 7 \\
D & 8 \\
F & 2 \\
G & 4 \\
\arrayrulecolor{black}\bottomrule
\end{tabular}

\end{document}