I notice that black leather livingroom chair in the black and white episodes,is replaced in the color episodes by a bright red leather chair.Another thing that appears to be replaced in the color episodes is the telephone.Where in the black and white shows their telephone was an older black model,we see a more up to date model in the color shows.Was there a particular episode where Steve decided to buy new furniture?Or was instantly replaced for the color shows?

The minor aesthetic changes you refer to came about due to the series switching networks at the start of the sixth season in 1965 which is when the show began being filmed in color. It was never mentioned in the storylines.

Never mind the furniture, how about the house! There were scenes where Katie was looking out her and Robbie's bedroom window, outlooking the front... But when Rob & Katie had the triplets and their bedroom was enlarged the outside of the house never changed! Also when Rob was in Peru, Katie & the triplets moved back in with the family... There was suddenly an extra bedroom across from Ernie's room. Katie's room had a room off it for the triplets! LOL! It's funny how TV sets change.

...during the entire run of the series (with the exception of the transition from black & white to color in '65 and the "Big Move to California" storyline in late 1967) because the series worked within a tight budget...especially when they had to film a season's worth of episodes in three months, to accomodate Fred MacMurray's availability. When the later episodes reflected Rob & Katie's growing family, they had to "add" another set without too much difference in the "stock shots" of the house outdoors.