How do i put fog in just one areaand change its color?

ive tried putting fog already but it shows up in the entire map and i wanted in just one of the areas of the map.

You might want to try to create a part in the area you want, then turning collision off, afterward making it invisible. Lastly, just press the plus sign on the tool, then add the instance "Smoke". Just play around with the properties afterward until you like it.