Hello 3DWinter,
I could be wrong in the original post, it doesn't seem to be that easy to "crop" the filtered points using current API.
You can, however, follow these steps, providing that you are not interested in preserving the results of the point cloud classification:
chunk.dense_cloud.setConfidenceFilter(4,255)
chunk.dense_cloud.selectPointsByColor((155,155,155), tolerance = 255) #will select all the filtered points
chunk.dense_cloud.assignClass(127)
chunk.dense_cloud.resetFilters()
chunk.dense_cloud.removeSelectedPoints(list(range(127)))