How to make Image Texture have white background and/or transparent background with a second white material behind it on the same object?

Question

I have a creature in Blender that I'm trying to texture the eye of. Right now the eye is a sphere with an Image Texture Material on it. The Image Texture is a .png of a pupil with a transparent background. When I put that as an Image Texture on a material, it shows up with a Black background. But I want the creature to have a white eye behind the pupil.

How do I toggle with the settings to achieve a White background behind the transparent-background pupil texture png? I can attach pictures of my setup if needed but I've barely touched the default settings.

Thank you.

Edit: Solved! Submitted answer below.

Answer 1

This answer helped me the most: Change color of PNG background (transparent to color)

Particularly using the MixRBG for Nodes (which btw are in 'Shader' window, not 'Nodes' window)

I copied the MixRBG part of this guy's setup. https://i.sstatic.net/kZOHk.jpg

Thanks all!