Use the result of FPeval to color a table cell

You could add the line

\xdef\resa{\resa}%%

to your code. But things still won't compile properly because your multiplier gets you out of the range from 0 to 1.

\documentclass{article}
\usepackage{fp,xcolor,colortbl}
\FPeval{\resb}{0.5}

\newcommand{\he}[1]{%
    \FPeval{\resa}{2 * #1}%
    \xdef\resa{\resa}%%
    \cellcolor[gray]{\resa}%
    #1 
}

\begin{document}

\begin{tabular}{| c | c | c |}
    \hline
    \he{0.1} & \he{0.2} & \he{0.3} \\
    \hline
    \he{0.5} & \he{0.8} & \he{1.0} \\
    \hline
\end{tabular}

\end{document}

To get a compilable version I wrote

\documentclass{article}
\usepackage{fp,xcolor,colortbl}
\FPeval{\resb}{0.5}

\newcommand{\he}[1]{%
    \FPeval{\resa}{2 * #1}%
    \xdef\resa{\resa}%%
    \ifdim\resa pt>1pt\relax
      \gdef\resa{1}%%
    \fi
    \cellcolor[gray]{\resa}%
    #1 
}

\begin{document}

\begin{tabular}{| c | c | c |}
    \hline
    \he{0.1} & \he{0.2} & \he{0.3} \\
    \hline
    \he{0.5} & \he{0.8} & \he{1.0} \\
    \hline
\end{tabular}

\end{document}

The following example solves the issue by expanding \resa before \cellcolor is expanded and looks at its arguments.

The second problem is, the range for the color model gray is between 0 and 1 inclusively. The values 0.8 and 1.0 exceed this, when multiplied by 2. Therefore the example checks the result and limits it to 1 if necessary.

\documentclass{article}
\usepackage{fp,xcolor,colortbl}

\newcommand{\he}[1]{%
    \FPeval{\resa}{2 * #1}%
    \ifdim\resa pt>1pt %
      \def\resa{1}%
    \fi
    \edef\processme{\noexpand\cellcolor[gray]{\resa}}%
    \processme
    #1%
}

\begin{document}
\begin{tabular}{| c | c | c |}
    \hline
    \he{0.1} & \he{0.2} & \he{0.3} \\
    \hline
    \he{0.5} & \he{0.8} & \he{1.0} \\
    \hline
\end{tabular}
\end{document}

And a version, where all the math is done by fp means. Also the text color is switched to white, if the background color is getting to dark to maintain readability.

\documentclass{article}
\usepackage{fp,xcolor,colortbl}

\newcommand{\he}[1]{%
    \FPeval{\resa}{max(0, min(1, 2 * #1))}%
    \edef\processme{\noexpand\cellcolor[gray]{\resa}}%
    \processme
    \FPiflt{\resa}{0.5}%
      \color{white}%
    \fi
    #1%
}

\begin{document}
\begin{tabular}{| c | c | c |}
    \hline
    \he{0.1} & \he{0.2} & \he{0.3} \\
    \hline
    \he{0.5} & \he{0.8} & \he{1.0} \\
    \hline
\end{tabular}
\end{document}

Still, there is room for improvement. The number result has lots of unnecessary zeros at both ends, which can be shortened, e.g.:

0.200000000000000000 ⇒ .2
0.400000000000000000 ⇒ .4
1.000000000000000000 ⇒ 1

The zeroes at the end can be removed by \FPclip to 0.2, 0.4 and 1.

Package thepdfnumber goes a step further and provides \thepdfnumber for shortening the decimal numbers at both ends to get .2, .4 and 1. Even better, \thepdfnumberNormZeroOne takes care of the range condition for the gray values:

\documentclass{article}
\usepackage{fp,xcolor,colortbl}

\usepackage{thepdfnumber}

\newcommand{\he}[1]{%
    \FPeval{\resa}{2 * #1}%
    \edef\processme{%
      \noexpand\cellcolor[gray]{\thepdfnumberNormZeroOne\resa}%
    }%
    \processme
    \FPiflt{\resa}{0.5}%
      \color{white}%
    \fi
    #1%
}

\begin{document}
\begin{tabular}{| c | c | c |}
    \hline
    \he{0.1} & \he{0.2} & \he{0.3} \\
    \hline
    \he{0.5} & \he{0.8} & \he{1.0} \\
    \hline
\end{tabular}
\end{document}

A simpler solution using expl3 and its powerful fp module.

\documentclass{article}
\usepackage[table]{xcolor}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\he}{m}
 {
  \cellcolor[gray]{ \fp_eval:n { min ( 2*#1, 1 ) } }
  #1
 }
\NewDocumentCommand{\hetest}{m}
 {
  \cellcolor[gray]{ \fp_eval:n { min ( 2*#1, 1 ) } }
  \textcolor{red}{#1 ~ -- ~ \fp_eval:n { min ( 2*#1, 1 ) }}
 }
\ExplSyntaxOff

\begin{document}

\begin{tabular}{| c | c | c |}
\hline
\he{0.1} & \he{0.2} & \he{0.3} \\
\hline
\he{0.5} & \he{0.8} & \he{1.0} \\
\hline
\end{tabular}
\qquad
\begin{tabular}{| c | c | c |}
\hline
\hetest{0.1} & \hetest{0.2} & \hetest{0.3} \\
\hline
\hetest{0.5} & \hetest{0.8} & \hetest{1.0} \\
\hline
\end{tabular}

\end{document}

The \hetest command also prints (in red) the argument and the computed color value, for testing purpose.

Variant 1

The argument is printed white if the grayness is below 0.5

\documentclass{article}
\usepackage[table]{xcolor}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\he}{m}
 {
  \cellcolor[gray]{ \fp_eval:n { min ( 2 * #1 , 1 ) } }
  \fp_compare:nT { 2 * #1 < 0.5 } { \color{white} }
  #1
 }
\ExplSyntaxOff

\begin{document}

\begin{tabular}{| c | c | c |}
\hline
\he{0.1} & \he{0.2} & \he{0.3} \\
\hline
\he{0.5} & \he{0.8} & \he{1.0} \\
\hline
\end{tabular}

\end{document}

Variant 2

The same as before, but the values are stored in fp variables; this may be convenient in case the computations are heavier, for performance reason.

Here I define syntactic sugar \__ecker_cellcolor:n for \cellcolor[gray]{...} so that with a variant we can expand the value at call time, using f expansion.

\documentclass{article}
\usepackage[table]{xcolor}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\he}{m}
 {
  \ecker_he:n { #1 }
 }

\fp_new:N \l_ecker_resa_fp
\fp_new:N \l_ecker_resb_fp
\cs_new_protected:Npn \ecker_he:n #1
 {
  \fp_set:Nn \l_ecker_resa_fp { 2 * #1 }
  \fp_set:Nn \l_ecker_resb_fp { min ( \l_ecker_resa_fp, 1 ) }
  \__ecker_cellcolor:f { \l_ecker_resb_fp }
  \fp_compare:nT { \l_ecker_resb_fp < 0.5 } { \color{white} }
  #1
 }
\cs_new_protected:Npn \__ecker_cellcolor:n #1
 {
  \cellcolor[gray]{ \fp_eval:n { #1 } }
 }
\cs_generate_variant:Nn \__ecker_cellcolor:n { f }
\ExplSyntaxOff

\begin{document}

\begin{tabular}{| c | c | c |}
\hline
\he{0.1} & \he{0.2} & \he{0.3} \\
\hline
\he{0.5} & \he{0.8} & \he{1.0} \\
\hline
\end{tabular}

\end{document}

Output of variants 1 and 2