Another approach to get the correct RGB values from the GW2 color API as Cliff Spradlin described over here: https://forum-en.guildwars2.com/forum/community/api/How-To-Colors-API/2148826

gw2colorsV2.php

<?php

// note: the array Syntax $foo = [...] is available since PHP 5.4, you may need to change to $foo = array(...)
// http://php.net/manual/en/language.types.array.php

function matrix_multiply($m1, $m2){
	$r = count($m1);
	$c = count($m2[0]);
	$p = count($m2);
	if(count($m1[0]) != $p){
		return false; //incompatible matrix
	}
	$m3 = [];
	for($i = 0; $i < $r; $i++){
		for($j = 0; $j < $c; $j++){
			$m3[$i][$j] = 0;
			for($k = 0; $k < $p; $k++){
				$m3[$i][$j] += $m1[$i][$k]*$m2[$k][$j];
			}
		}
	}
	return ($m3);
}

function compositeColorShiftRgb($hslbc, $base){
	// colors from the response .json -> material
	$h = ($hslbc['hue']*pi())/180;
	$s = $hslbc['saturation'];
	$l = $hslbc['lightness'];
	$b = $hslbc['brightness']/128;
	$c = $hslbc['contrast'];

	// 4x4 identity matrix
	$matrix = [
		[1, 0, 0, 0],
		[0, 1, 0, 0],
		[0, 0, 1, 0],
		[0, 0, 0, 1]
	];

	if($b != 0 || $c != 1){
		// process brightness and contrast
		$t = 128*(2*$b+1-$c);
		$mult = [
			[$c,  0,  0, $t],
			[ 0, $c,  0, $t],
			[ 0,  0, $c, $t],
			[ 0,  0,  0,  1]
		];
		$matrix = matrix_multiply($mult, $matrix);
	}

	if($h != 0 || $s != 1 || $l != 1){
		// transform to HSL
		$multRgbToHsl = [
			[ 0.707107, 0,        -0.707107, 0],
			[-0.408248, 0.816497, -0.408248, 0],
			[ 0.577350, 0.577350,  0.577350, 0],
			[ 0,        0,         0,        1]
		];
		$matrix = matrix_multiply($multRgbToHsl, $matrix);

		// process adjustments
		$cosHue = cos($h);
		$sinHue = sin($h);
		$mult = [
			[ $cosHue * $s, $sinHue * $s,  0, 0],
			[-$sinHue * $s, $cosHue * $s,  0, 0],
			[            0,            0, $l, 0],
			[            0,            0,  0, 1]
		];
		$matrix = matrix_multiply($mult, $matrix);

		// transform back to RGB
		$multHslToRgb = [
			[ 0.707107, -0.408248, 0.577350, 0],
			[ 0,         0.816497, 0.577350, 0],
			[-0.707107, -0.408248, 0.577350, 0],
			[ 0,         0,        0,        1]
		];
		$matrix = matrix_multiply($multHslToRgb, $matrix);
	}

	// apply the color transformation
	$bgrVector = [
		[$base[2]],
		[$base[1]],
		[$base[0]],
		[1]
	];
	$bgrVector =  matrix_multiply($matrix,$bgrVector);

	// clamp the values
	$rgb = [
		floor(max(0, min(255, $bgrVector[2][0]))),
		floor(max(0, min(255, $bgrVector[1][0]))),
		floor(max(0, min(255, $bgrVector[0][0])))
	];

	return $rgb;
}

?>