Hi, and welcome!
The unit and HumViewer are not operated the same way. HumViewer is a free 3rd party program that is not supported by Humminbird.
In HumViewer:
Change color: right-click on the image and select Color schemes.
Zoom: click the Utilities button, select Zoom and then position the cursor on the view and mark the area you wish to enlarge.
Regards,
Rickard