Raymond, you're correct insofar as the actual address fields in the Household area does gray out when the Personal Address checkbox is checked, indicating and alternate address.
Kang, take a look at the February 2017 Release notes
http://help.elexio.com/database/release-notes/2017-q1 Secondary Households section, where it says "We added the ability to specify a secondary household for each person to be used when sending an email or SMS message to the household head and spouse from the Communications area." This change means you can now assign Personal addresses individually anybody in a Household.
The overall gray background color of the Household area was a cosmetic change that seemed like a good idea at the time, but met with some push-back by Elexio Deluxe users during the February Release Day webinar. Keep an eye out for that to potentially change back to a white background like we're all familiar with.