RE: RED BLUE OF BLACK - AHHHHH15 Mar 2018 13:46
It's more obvious on SETS, where everything is a strict trade (buyer and seller instantly matched).
With MM-traded shares, the trade is (usually) via an MM, so there's a buy/sell direction relative to the MM (ie, buy from MM, sell to MM).
The Exchange could have added a bit to say which way it was for a given trade, but didn't.
Since the crowd likes to know, sites like this simply guess, depending whether it's above or below the mid price, but when the advertised spread is big and the actual spread is much less within that, it's fairly common for the prices to sit one side of mid or the other, and you have to look at the actual range of prices to work it out yourself (which the sites could also do, but I've never seen one that did).
cunning MMs sometimes buy and sell at mid, leaving everyone none the wiser.