\def\cals@framecs@width{0pt}
\def\cals@cs@width{1pt}
\def\cals@bgcolor{red}

\let\cals@borderL=\relax
\let\cals@borderR=\relax
\let\cals@borderT=\relax
\let\cals@borderB=\relax

\colwidths{{100pt}{200pt}}
\cals@decor@begin

\cell{aaaa}
\showbox\cals@current@cs
\show\cals@lastWidth
\def\cals@borderL{2pt}
\def\cals@borderT{3pt}
\def\cals@borderR{4pt}
\def\cals@borderB{5pt}
\def\cals@bgcolor{green}
\cell{b}
\showbox\cals@current@cs
\show\cals@current@rs@above
\show\cals@current@rs@below
\show\cals@lastWidth

\cals@decor@end\cals@borderR
\show\cals@current@rs@above
\show\cals@current@rs@below