Because my pictures are so terrible, people have asked me in the past how I "choose" which pictures lose their colors and which ones do not. For me personally, as long as turning them into monochromes make the pictures worse than they were before, then I am ok with that. What I often do is to do a trial run of BW version of a picture to see how badly I hate it. If I do, then it is a go.
If you read photography books and magazines and other bullshit, many people have different approach about BW photography. Some exclusively shoot in BW, while others shoot roughly 10 percent, 20 percent, 30 percent and so on, of course. Many claims that before they shoot this particular picture they know that they are becoming BW.
It seems to me that when a color pictures "almost" look monotone, then it is time to try BW version of it. I feel like I have more control of the situation, where turning the picture worse than before becomes much easier, more intuitive, and rewarding than ever before.
FA 31mm ltd
1/250 @ f/4.5
By the way I think that K10D's sensor is just superb for BW photography, compared to other brands in the same category, and hold its own admirably when stacked up against higher price DSLR. So I really should not be using it, really.