What do the green and red volume tags on the Yahoo Summary Chart mean?

You are correct that if the Volume bar is red the price has decreased and if it's green the price has increased. But you need to pay attention to the time interval across which Yahoo is making that distinction.

The price may have risen or fallen over the entire day but those Volume bars on the summary page are representative of half hour intraday slices. So the volume bar is red because for whatever half hour slice you are hovering over, the price at the beginning of that half hour period was lower than the end. The price on the day may have ended up, but it fell for that period.

It's much easier to see this if you open up a full screen chart and change the chart type to Candle or Colored Bar. The volume bars will line up to the Candle/Colored Bar periods and you can clearly see where a stock price ended in a period versus when it began that period and the volume bar should be colored appropriately.