The new "specially calculated" Leica color filters for B&W are now in stock at Leica Store Miami and at PopFlash (yellow, green, and orange available in E39 and E46). This new set of filters were announced together with the M Monochrom Typ 246 camera. Additional information is available here:
"By modifying the conversion of colours to greyscale values, yellow, green and orange lens filters offer new opportunities for the exploration of creative effects with light and contrast. This means that the colour of the filter in the original scene becomes brighter and its complementary colour darker. This is effective for creating unusual moods in landscapes, portraits and other genres of photography. At the same time, multilayer coating reduces reflections and ensures high transmission without vignetting."