\cals@withColorII{black}{blue}\iftrue
 \PackageInfo{test}{user color: \cals@color }
\else
 \PackageInfo{test}{fail}
\fi

\cals@withColorII{black}{}\iftrue
 \PackageInfo{test}{fail}
\else
 \PackageInfo{test}{(user) no color}
\fi

\cals@withColorII{black}\relax\iftrue
 \PackageInfo{test}{default color: \cals@color}
\else
 \PackageInfo{test}{fail}
\fi

\cals@withColorII{}\relax\iftrue
 \PackageInfo{test}{fail}
\else
 \PackageInfo{test}{(default) no color}
\fi