Nicely formatting booktab table with multiple columns

Question

I have a pretty ugly table that I would like to format nicely. I have seen in LaTeX table cell with a diagonal line and 2 sub cells how I can include \slashbox and in Parametrize shading in table through TikZ how I can colour code a table. However, this makes the table look pretty ugly. How can I turn my current table into something as nice and professional looking as Vertical table lines are discontinuous with booktabs, while also implementing conditional formatting?

EDIT:

I now have a nice table thanks to Mico, but I have decided to merge two tables into one. I'm trying to get another multicolumn below DIRINT, DISC and Erbs like so:

Edit edit: Fixed it!

Answer 1

I doubt that making a table look gaudy -- say, by assigning dominant, bright colors to the table cells -- actually makes the table more readable, as opposed to just making it stand out visually. Providing more structure to the header material and providing some kind of visual rhythm to the body of the table should be at least as successful.

\documentclass{article}
\usepackage{booktabs,siunitx}
\sisetup{output-decimal-marker={,}}
\begin{document}

\begin{table}[h]
\centering
\caption{MAE for 48 irradiance transposition pathways}
\label{tab:MAE results}
\begin{tabular}{@{} l *{3}{S[table-format=2.2]} @{}}
    \toprule
    Decomposition & \multicolumn{3}{c@{}}{Transposition} \\
    \cmidrule(l){2-4}
    & {DIRINT} &  {DISC} &  {Erbs}   \\
    \midrule
    Reindl                  &   8,11 & 16,79 & 21,79   \\
    Hay \& Davies           &   8,11 & 16,75 & 21,75   \\
    P. Albuquerque   1988   &   8,33 & 17,39 & 23,34   \\
    P. Phoenix 1988         &   8,34 & 17,39 & 23,22   \\
    P. Sandia composite 1988&   8,39 & 17,38 & 23,16   \\ \addlinespace
    King                    &   8,41 & 16,75 & 20,52   \\
    P. Cape Canaveral 1988  &   8,50 & 17,55 & 22,53   \\
    P. USA composite 1988   &   8,51 & 17,51 & 22,98   \\
    P. Osage 1988           &   8,55 & 17,87 & 24,03   \\
    P. All sites composite 1990&8,58 & 17,61 & 22,79   \\ \addlinespace
    Isotropic sky           &   8,59 & 16,20 & 20,70   \\
    P. Albany 1988          &   8,65 & 17,54 & 22,72   \\
    P. Elmonte 1988         &   8,65 & 17,61 & 23,26   \\
    P. All sites composite 1988&8,74 & 17,79 & 23,06   \\
    P. France 1988          &   9,23 & 18,17 & 23,02   \\ \addlinespace
    Klucher                 &  10,38 & 20,02 & 23,53   \\
    \midrule
    \multicolumn{1}{r}{Column averages} 
                            &   8,63 & 17,52 & 22,65   \\
    \bottomrule
\end{tabular}
\end{table}
\end{document}