There are 3 different types of CheckBoxes in Excel: UserForm, Form Control, and ActiveX CheckBoxes. This answer will show you how to change the interior color for each of these CheckBox types manually and using VBA.
UserForm CheckBoxes are placed on top of a UserForm you design in the VBA Editor. To change the interior color of UserForm CheckBox, you adjust the BackColor property. You can do this by hand or you can do it with VBA, like this:
Me.CheckBox1.BackColor = RGB(255, 0, 0)
Change "Me" to the name of the UserForm if your code isn't placed in the UserForm whose checkbox you're wanting to change.
Form Control CheckBoxes
Form Control CheckBoxes are located under Insert > Form Controls on your Developer Tab in Excel and are placed within a spreadsheet itself.
You can change the interior color of a Form Control CheckBox manually by right-clicking it, selecting "Format Control" and selecting a Fill Color.
There are 2 common ways to change the interior fill color of a Form Control CheckBox using VBA. This macro presents both, with the second method commented out:
ActiveSheet.CheckBoxes("Check Box 1").Interior.Color = RGB(255, 0, 0)
'ActiveSheet.Shapes("Check Box 1").OLEFormat.Object.Interior.Color = RGB(255, 0, 0) 'Alternate Method
ActiveX CheckBoxes are the 3rd type of CheckBox you can make in Excel. Like the Form Control CheckBoxes, the ActiveX CheckBoxes also go on your spreadsheet. They're located under Insert > ActiveX Controls on your Developer Tab.
You can adjust the interior color manually by accessing the BackColor property of the control, just like with UserForm Controls. To do this, make sure "Design Mode" is clicked on your Developer Tab.
Once it's clicked, right-click your checkbox and click Properties. This will bring up a new screen where you can adjust the BackColor property:
You can change the BackColor property using VBA with one of these 2 methods:
ActiveSheet.CheckBox1.BackColor = RGB(255, 0, 0)
' ActiveSheet.OLEObjects("CheckBox1").Object.BackColor = RGB(255, 0, 0) 'Alternate Method
Notice the Name of the CheckBox matches the Name Property of the ActiveX CheckBox.