The Shadows' personal invisibility might be more a trick of the mind, akin to Kosh's appearance to other beings.
I hadn't actually thought about the Shadow's seeming invisibility as being a mental thing. That could also play into how in "In The Shadow Of Z'ha'dum" Talia, being a telepath, is able to catch a glimpse at the two shadows around Morden.
However, it wouldn't fit with Zack's later being able to scan on different light wavelengths with the cell's camera to momentarily reveal the Shadows in Morden's cell.
The past few days I've been thinking up an alternate theory about how the Shadows can seemingly turn invisible, as well.
I've been kind of perplexed as to how the Shadows can walk around all invisible (should it be they're just able to refract light to make them invisible) and not bump into people all over the place. In the above mentioned Talia in the hallway scene, how was it that they didn't physically run into the Shadows in that relatively small hallway? Another similar example, how did the Shadow get into the Centauri throne room in "Into The Fire" when Morden was brought in without banging into the doorway or the guards?
So, thinking about this for a while, I thought, perhaps they're not really invisible. Maybe the Shadows have some way of creating a very localized space-hyperspace portal and kind of sit the fence when they're "invisible." In other words, they are partially in hyperspace and partially in real space when they're seemingly invisible, and that allows them to occupy the same space as something else in regular space. That would keep them from bumping into people all over the place as they come and go.
Yeah, I know it's far-fetched, but I haven't yet thought of any other decent explanation for why they're not bumping into people all the time when they're around Morden.