There are three items that share the same code and are the signature blue out of the box.
The Advanced Search
The Hide Map
The Show Map (which appears if the Hide Map is enabled)
These three options populate on the default search page. The advanced search may not if you are using the Streamlined Search as your consumer-facing search interface.
If you'd like to change the color of these functions, you may use this code:
.sidx-container .sidx-collapse-button {
color: #hexcode !important;
}
Remember, place this code in the custom CSS box under Settings > Design. Clear your caches and check your web page in an incognito browser to see the change!