podyx, thanks for the explanation.
Check the 1.5k volume bar on this chart. You can see the price going up in almost a straight line during that trading period. I assume that means the ask side got eaten into. If the price had gone down in almost a straight line I would assume the opposite.
You're looking at (at a glance) 15 minute bars? One bar != 1 transaction. Try 5, 3, or 1 minute resolutions here:
https://bitcoinwisdom.com/markets/bitfinex/btcusd, and you'll see that 1.5k bar get broken down into a bunch of little ones.
As podyx pointed out, it's not that simple. You'll see green 1k bars from 499 market sells/501 market buys, for instance (1 BTC "ate into the ask side"). If the ask side is thin, you'll see the price shoot up.