Hollow blocks work exactly the same as regular blocks, except for the fact that they cannot activate plates or switches. Appearance should be the outline of a box, thick enough to be visible and look nice (so not exactly an "outline"), but so that you can see a plate/switch "inside" of the box when the box is on top of it. I would suggest white or light gray for the color, experiment with that and pick whichever turns out to look nicer.
Hollow blocks work exactly the same as regular blocks, except for the fact that they cannot activate plates or switches. Appearance should be the outline of a box, thick enough to be visible and look nice (so not exactly an "outline"), but so that you can see a plate/switch "inside" of the box when the box is on top of it. I would suggest white or light gray for the color, experiment with that and pick whichever turns out to look nicer.