<?php

$arr = [
	[
		'COLOR'    => 'Red',
    	'WEIGHT'   => 20
	],

    [
        'COLOR'    => 'Red',
        'WEIGHT'   => 25
    ],

    [
        'COLOR'    => 'Red',
        'WEIGHT'   => 30
    ],

    [
        'COLOR'    => 'Green',
        'WEIGHT'   => 20
    ],

    [
        'COLOR'    => 'Green',
        'WEIGHT'   => 25
    ],

    [
        'COLOR'    => 'Green',
        'WEIGHT'   => 30
    ]
];

$out = [];

foreach ($arr as $a) {
    $out[$a['COLOR']][] = $a['WEIGHT'];
}

foreach (array_map(function($a){ return join(', ', $a); }, $out) as $k => $item) {
    echo "$k $item\n";
}