All question I have looked to doesn't help me to solve my problem. Here's my code :
\documentclass[a4paper,10pt]{article}
\usepackage[utf8]{inputenc}
\usepackage[frenchb]{babel}
\usepackage{color}
\usepackage{colortbl}
\usepackage{longtable}
\usepackage{lmodern}
\usepackage[table]{xcolor}
\usepackage{multirow}
\usepackage{pdflscape}
\usepackage{hyperref}
\newcommand*{\thead}[1]{\multicolumn{1}{|c|}{\bfseries #1}}
\begin{document}
\newcommand{\mc}[3]{\multicolumn{#1}{#2}{#3}}
\begin{landscape}
\begin{longtable}{|>{\raggedright\arraybackslash}p{3cm}|p{7cm}|c|p{7cm}|}
\hline
\cellcolor{gray!30} \centering \bfseries Flux de\par substance étudié &
\cellcolor{gray!30} \centering \bfseries Mécanisme \par environnemental &
\cellcolor{gray!30} \bfseries Unité &
\multicolumn{1}{|p{7cm}|}{\cellcolor{gray!30} \centering \bfseries Remarques\par(RECORD)} \\
\cline{1-4}
\cellcolor{green!50} & %
\cellcolor{green!50} \'Emission de CO2 liée à l'épandage de chaux &
\cellcolor{green!50} &
\\
\cline{2-2}
\cellcolor{green!50} \multirow{-2}{*}{CO2} &
\cellcolor{green!50}![enter image description here][1] \'Emission de CO2 liée à l'épandage d'urée &
\cellcolor{green!50} \multirow{-2}{*}{$kg/ha$} &
\\
\hline
\end{longtable}
\end{landscape}
\end{document}
Here's what I get :

Table cell color overlaps cell border but I want the borders to be always visible. The answers given to questions I read doesn't helped as the people who give the solution doesn't explain well the code they give.
How can I solve my problem?