This game was played between me (blue) and another Top-10 player (yellow). It should be clear from looking at the finished game that something went wrong for Y/G – not only did B get big bonus, but even R did better than both Y and G. Y/G was at least successful in scoring well, as 152 is usually a winning score, but failed to prevent B/R from scoring REALLY well.
In games like this, it is often easy for Y/G to think that the game is a disaster early on, but more often than not things are not as bad as they seem. When the final result is this lopsided, often it is the case that the losing side made compound-errors: errors that occurred AFTER this player felt he was in a losing position that made the position even worse. This is certainly the case here.
The first time that Y felt bad in this game was after the Yellow F. Following the F, Red plays X and it becomes clear that B/R is going to gain a DCCZ on the lower-right side of the board. In this case, I would not consider the Yellow F a mistake: a move to the bottom to prevent the B/R DCCZ would allow the Blue Z5 top right, and let B/R get a DCCZ in the top-right anyway.
That said, things don’t look good for Y/G after three more moves, as BR has a decently sized zone in the bottom right that it seems will be defensible.
So maybe Y/G made a mistake before the Yellow F, but either way the position from this point is not nearly as bad as the final score is. Yellow must have made some further mistakes along the way.
The problems begin to compounded on the next Y move. Yellow plays T, a cut move to rush to fight with B. This is a major error since Y will be tied up elsewhere on the following turn or two (dealing with R) so B will win this fight. A kiss move would be a much better choice on this turn.
To see why yellow will be tied up, refer to the original image. Yellow will play P on the following move, which is necessary to defend against R if R plays the 1 early. As it turns out, R chooses to hold off on the 1 because Y is able to defend, but if Y didn’t play the P, the Red 1 would certainly have caused even more problems for Y/G.
A few turns later, Y will make another mistake. The Blue T4 threatens to destroy the small DCCZ that Y/G has been able to create in the bottom left. Y is worried about getting completely killed on the top (which he worries would be especially bad since this would take some pressure off of B – then it looks like B bonus is possible given the DCCZ for B/R). So Y chooses flight, and plays the I5 into the top left rather than defending the DCCZ in the bottom.
Here is the problem: B will play the O4 to kill the Y/G DCCZ, and many of the points Y gains in the top are effectively lost in the bottom. In addition to this, red will be able to play the 1 from the L5, and also gain some points in the bottom left. Since there are lots of G corners in the top left, Y doesn’t need to put pressure on B (G can do this). When you combine these reasons, it becomes clear that Yellow Z4 to defend the bottom left would be a much better play than I5.
A good rule of thumb is this: When deciding between which of the opponent’s zones to play in, all other things being equal, play in the zone where two of the opponent’s colours can play instead of the zone where only one can play. In this case, both B and R could play in the bottom left, but R would never be able to play in the top left, so it was more important to play in the bottom left.