I have never seen yellow numbers. I've only seen green signifying an increase and red signifying a decrease.
There are no useless buildings in this game, because they don't cost maintenance and only very rarely some resources, the only limiting factor is production time and tech/city level. There are very few buildings (black market for example) that actually increase a certain aspect of the city while at the same time decreasing another one, rendering them useless in certain situations. Just keep in mind that almost all improvements are part of a mini production tree and when you build the next improvement it is basically the next stage in the tree.
If the numbers in front of and behind the arrow stay the same, it indicates that the building you are upgrading from already increased that number and the one you want to build does not change it. It increases another aspect of the city and unlocks the next stage in the mini tree.