Use different colors for every another grep

You could add the following to your .bashrc file:

my_grep() {
  if $GREP_USE_FIRST_COLOR
  then
    export GREP_USE_FIRST_COLOR=false
    export GREP_COLORS='ms=01;31:mc=01;31:sl=:cx=:fn=35:ln=32:bn=32:se=36'
  else
    export GREP_USE_FIRST_COLOR=true
    export GREP_COLORS='ms=01;33:mc=01;33:sl=:cx=:fn=35:ln=32:bn=32:se=36'
  fi
  grep --color=auto "$@"
}
alias grep=my_grep

This will alter the match highlighting color of subsequent grep calls in an interactive shell between bold red (ms=01;31) and bold yellow (ms=01;33).

See the grep manual for more information on how to use GREP_COLORS to set the particular highlighting colors you like.