They were each Top 30 shows their first seasons, the Addamses at #23 and the Munsters at #18. The Nielsen ratings charts signal that each show fell out of the Top 30 in their sophomore seasons, all though it's a toss-up as to whether they were winning their time slots or not. (I haven't found the information to be precise.)
I think it would have been interesting to see them each in color, and really in color, not these colorized attempts. Maybe the networks felt that type of satirical black humor had simply ran its course, and therefore pulled their plugs. If they had of went to color, I'd almost bet that we wouldn't be discussing if it should have switched, but instead how it changed when it switched.
It would have, as I theorize, brought forth a Andy Griffith-type situation, debating the pre- and post-B&W episodes, especially if the each had two B&W and two color seasons. Aside from the shows being different simply because their in color, I can't help but think they'd tweak something else and the feel of the show would have shifted.