If I've found the correct setting (on the Files tab of the Preferences window), the setting "Add tag, color class and rating to IPTC/XMP" is set to "Only when updating IPTC/XMP". The color classes are being applied to the images from the contact sheet using one of the numbers (e.g., 2, 3, or 4).
Also, this is happening even when images are imported without applying any IPTC template.
I've just tried the same workflow with PM 5 (the RC), and it does the same thing. If a color class is set prior to editing, the color class remains after the image has been edited. Is there any way to turn this behavior off?
[Sorry if this should be provided elsewhere] Also, I've looked at the PM 5 setting for color classes, and set the stationary to include color class (of no color class), and applied that to an edited image (as I would to caption and transmit images usually, within the caption window, pressing the apply stationary pad button). The no color class from the stationary did not override the color class set on the image. I've tried the same thing with the star ratings, and that doesn't seem to have any effect when applying stationary to the image. PM works as you've described only if selecting images on the contact sheet and then applying the stationary to all selected (or from the apply to selected button in the stationary pad itself).