First of, you will need to head over to your admin dashboard, and collapse the Advanced Menu to find the Widgets tab.
Once you are on the Widgets page, on your right side you will find your active widgets called Widgets for Shortcodes which are controled through shortcodes.
Now as you open that tab, you will see various widgets that are active, and can vary from site to site, the widget you need to find is called Agentfire Quick Search.
Open the Agentfire Quick Search tab, and scroll all the way down until you see the option Button Color.
As you click there, the color palette will appear where you can adjust the color at your will. After you are done, make sure to click on the Save button to save the changes you made.