IMUGENE CHART. TA only, page-29073

You are correct, there is always a buyer and seller for every transaction. With these type of indicators however (you can get them on a chart as some sort of 'volume/price' display) it is taken as the side who triggers the trade to happen. For example, if it is sitting at 10/10.5 all day, anyone who sells into 10 would go into the red for 10, anyone who buys into 10.5 would go into green for the 10.5 line. Then later in day, as an example price moves to 10.5/11. Green goes to 11 level when someone buys into the 11c depth and now red to those who sell into 10.5

This can show overall sentiment by those who are initiating a trade as a buyer vs seller. Eg: when a price starts to run, you will see mostly green as each line is wiped. 50/50 split is more about ranging markets.

I would suggest the grey is for centre point trades, as there is no way to know who initiates (sellers and buyers are matched up off-screen).

What is confusing with yours however is the green 10c line. Price was never 09.9/10 for there to be buyers into the 10 level. The only thing I can think of is it had something to do with the open auction this morning.

Edit:

I just saw this is ticks volume, so it is not a direct relationship of volume, but rather the relationship between individual trades (ticks). The above still stands.