[Fix #748] Set grep color for BSD and GNU

modules/utility/init.zsh

@@ -104,7 +104,9 @@ alias sl='ls'            # I often screw this up.
 
 # Grep
 if zstyle -t ':prezto:module:utility:grep' color; then
-  export GREP_COLOR='37;45'
+  export GREP_COLOR='37;45'           # BSD.
+  export GREP_COLORS="mt=$GREP_COLOR" # GNU.
+
   alias grep="$aliases[grep] --color=auto"
 fi