Whenever you see city centre pictures from the 50s/60s lots of buildings look darker and dirty due to what was in the air at the time yet the Lewis's building looks just lime today really clean
A lot of the older buildings in the city centre had not been cleaned in over a hundred years. That's a century or more of coal soot, traffic fumes, smog and the rest. So at that time, they were all dirty.
In comparison, the new Lewis's building was only rebuilt after the war, in the late 40s/early 50s.
Also take into effect, the clean air act that was brought in in the mid 1950s, which over the next twenty years or so, to the time that picture was taken, had reduced the amount of crap getting pumped into the atmosphere.
So at the time of this picture. the Portland stone on the new Lewis's was only about 20 years old, as apposed to the century of soot on the others.
It still makes me laugh, and cringe at the same time, when you see pictures of say, the Liver Buildings, St.George's Hall etc. from the mid 70s or earlier. You'd swear blind they were built from black stone.
Edit: The Lewis's building was also sand blasted/chemically cleaned within the last ten years as part of the conversion and refurb.