Essential Sudoku DS: a long way down a path

I'm now on puzzle 761. There have been a few recently where I've had to use other colours to test out whether a block can go somewhere and whether that would cause any contradictions later on. There was one puzzle which took me 50 minutes to do, because I went down two such dead ends and had to delete all the alternative colours.

The hardest puzzles are those where the picture lies along a diagonal of the square. You end up with only a few coloured blocks in each row and it's hard to start, but once started it's OK to finish. Puzzle 755, a picture of fish bones, was really difficult, because as well as the backbone lying diagonally down the middle, there were random scatterings of black for the ribs as well.

In fact, it looked like this: