The hotspots part of the wiki activity page is a place which displays which page has been edited by more then a single user in the past 3 days. Most people are not aware of the fact that this is also a part of the wiki which can be adjusted to your liking.

The colors of the different hot spots can be adjusted using the wikifile MediaWiki:Wikia.css

This can be done with using the following CSS line. However, each 'spot' has a specific color. They are called severity. For example, the most edited page has severity number 1 and is influenced like this:
<br />.HotSpotsModule li .hot-spot-severity-1 {<br />background-color: #CC0033;<br />}

:Change the color where it says background-color. You can change 1, 2, 3, or 4 for the other "hotspot" colors.

Do notice that changing Mediawiki:Wikia.css does influence only the pages using Oasis skin.

The colors used are Hex HTML color codes. A possible website which could help you easily identify what color code to use is found over here