No it's not a bug. Whether you can change the colour of a house or not depends on what kind of house it is. If you do it to one that has been placed as a varicolour symbol it will work. If not, it won't.

I think you mean map units, not scale. All city maps are the same scale - which happens to be the same scale as a dungeon map - 1 foot to one map unit.

As to the correct size, you will probably be a week or so discussing that one with everyone else, because the size of a city is only party dictated by the size of the population. If space is cramped cities are also cramped and tend to have more taller buildings than a city with the same population built in the middle of a vast plain. There are also cultural issues to consider. What is the accepted maximum number of families per household. In the UK just a couple of hundred years ago at the dawn of the Industrial age it was quite normal for 2-3 entire families to live in a single room of a multi-level building where they had come in from the country to find work in the new factories. These days people would gasp in horror at the very idea of it. We live very insular lives, and there are many who live as single adults with a whole flat to themselves.

There are many other considerations, but you get the idea. It's very much up to you how big you make the city, but useful to count each building by how many people you would expect to be living in it.

You've done a great job :)

To be able to save new settings in FT3 you need to have opened it by right clicking the icon and picking "Run as administrator" This is because the folder FT3 wants to save the settings is a protected system folder and you need to have administrator access to save in it.

Ralf did a Live Mapping session on this just last week here:

Hi Jason :)

I'm much more of a City and overland mapper, and have only once used Cosmographer, so I'm probably not the best person to answer your main question. I just wanted to encourage you to create new assets. After all - once you've done it once you won't ever have to do it again, no matter how many maps you make.