Could also simply be a viewport bug. Houdini's viewport is notoriously buggy with packed prims, especially with colours. Sometimes it'll display, sometimes it wont.
You can try appending an unpack to see if the colour is there, if so you can swear, log a bug with sidefx, know thats how you do it from now on. It should render properly with mantra/karma, and can be made to work with mild effort with other renderers.