I've been noticing that the focus read out on the LCD turns from white to grey at certain tight zoom levels. What does this signify?

OK. No answers as of yet (sigh)... I'm guessing it has something to do with zooming past the current focus point. ???

Sorry about the wait. If there's not enough light then there isn't
enough contrast for the AF sensor to use in order to accurately
read the focus distance. This might be one reason why you're
seeing that.

Also, when focusing manually, yes it is true that increasing the focal
length beyond the current focus point will throw off critical focus. Use
the Push AF button to reset critical focus -- the focus distance readout
should reappear.