In setting up a game playing program to allow easy addition of patterns (see X -> do Y), I discovered the complete list of patterns for this puzzle:

1. Find a square that, if you plant a tree there, will cause the elimination of at least one color. Mark that square as unavailable.
2. Repeat step 1. until solved.

That's it :)

Here's the code for it:

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1207779

use strict;
use warnings;

print local $_ = my $grid = <<END, "starting\n";
GRBBB
GRBBW
ORBBW
OOOWW
OOOOW
END

my $N = $grid =~ /\n/ && $-[0];
my $n = $N - 1;

sub clear # the no longer available squares
  {
  my $pick = qr/[a-z]/;
  local $_ = shift;
  1 while s/\w(?=.*?$pick)/-/ + s/$pick.*?\K\w/-/  # row
    + s/\w(?=(?:.{$N}.)*.{$N}$pick)/-/s          # column
    + s/$pick(?:.{$N}.)*.{$N}\K\w/-/s            # column
    + s/$pick.{$n}(..)?\K\w/-/s                  # lower diagonals
    + s/\w(?=.{$n}(..)?$pick)/-/s                # upper diagonals
    ;
  return $_;
  }

sub missingcolor { $N > keys %{{ map +($_, $_), shift =~ /\w/g }} }

while( /[A-Z]/g ) # mark square to '-' if tree there causes a missing 
+color
  {
  missingcolor( lc clear( "$`\l$&$'" ) ) and
    $_ = "$`-$'",
    print $_, ' ' x $N, " mark ",
      $-[0] % ($N + 1), ',', int $-[0] / ($N + 1), "\n"; # x,y coords
  }

print s/[A-Z]/$&/g == $N ? "\nSolved!\n" : "Failed\n";
[download]

It prints a grid for each step of the solution (slightly more than 120 lines).