You would have the same results with 'light.bluekeyword.com" as you would with "bluekeyword.com/light". Detailed URLs are ideal for maximizing Keyword Exposure.

If you market your site right, the extra WEIGHT on a folder will help boost the rank of your main domain, however when it comes to a subdomain, your pagerank would be ZERO and no "link love" trickles down.

You should only use a subdomain if needed to seperate your site or for a completely different theme to your site. If it's the same layout and same information related to your domain name, you should be using folders in order to keep your pagerank and authority higher.