Here's a variant which can be used if you always have people who build the same set of buildings (eg the double purple, then the 3 producing grey buildings), which can make games a bit similar. It also reduces thinking time slightly as there are fewer options in what to buy.
In the first round, shuffle the brown buildings, and draw 3 face up, do the same with the grey buildings, and set the rest of the grey and brown buildings aside. The Blue buildings should all remain visible as should the green buildings.
Only these brown and grey buildings are available for purchase.
At the end of any round, if there are less than 4 brown buildings available and no brown building was purchased this turn, then one is added to those available from those set aside.
At the end of any round, if there are less than 4 grey buildings available and no grey building was purchased this turn, then one is added to those available from those set aside.
If after a grey or brown building is purchased, there are less than 3 buildings available of that colour, then one of that colour is immediately drawn from those set aside and added to the available area.