SEO Google Keywords Tool Question

I am looking at keywords for my site on Google adwords keywords tool. When I put in a keyword phrase and it gives me a result, I get a bar that is either all white, all green or somewhere in between that are advertisers using that keyword. I also get  the number of Monthly Volume of searches for the word or phase. My question, what am I looking for; full green bar, half green half white a bar, full white bar? And volume; High volume, mid volume or low volume (68,000,000-246,000-14,800)? What in general should I be looking for in a keyword phrase in these results, what is Good, Mid, Bad?

You would do better asking this in the google adsense forums.