private function _addColor($rgb) {
		if (!isset($this->_colors[$rgb])) {
			if (count($this->_colors) < 57) {
				// then we add a custom color altering the palette
				$colorIndex = 8 + count($this->_colors);
				$this->_palette[$colorIndex] =
					array(
						hexdec(substr($rgb, 0, 2)),
						hexdec(substr($rgb, 2, 2)),
						hexdec(substr($rgb, 4)),
						0
					);
				$this->_colors[$rgb] = $colorIndex;
			} else {
				// no room for more custom colors, just map to black
				$colorIndex = 0;
			}
		} else {
			// fetch already added custom color
			$colorIndex = $this->_colors[$rgb];
		}

		return $colorIndex;
	}