<?php
$message="I have a [red][car]";
echo preg_replace('/\[([^\]]+)]\[([^\]]+)]/',"<b style='color:$1'>$2</b>",$message);
PD9waHAKCiRtZXNzYWdlPSJJIGhhdmUgYSBbcmVkXVtjYXJdIjsKZWNobyBwcmVnX3JlcGxhY2UoJy9cWyhbXlxdXSspXVxbKFteXF1dKyldLycsIjxiIHN0eWxlPSdjb2xvcjokMSc+JDI8L2I+IiwkbWVzc2FnZSk7