Hi Swati Kharbas,
This is a cool one. You need to do 2 things;
1. Set your expression with the help of set analysis
Your dimension is City so your expression should ignore that (otherwise only the selected values are shown)
SUM({$<City=>}Population
2. Set your color codes:
The expression for your color should react on the selections. I mostly use
IF ONLY(City) = ONLY({1}, RED(),BLACK()).
Because City is the dimension it also works with multiple cities selected because it looks at every row. Then it compares this selected cities to all the cities. With {1} all selection are ignored.
