my product is using 100+ white icons on a transparent background. How can I save the icons with a color background, so I can see the icons in folder view- but maintain the cutout of the icon? Can I use a alpha matte or a mask? Without a color background, the white icon on a white background looks blank and impossible to view
I don't believe this is possible the way you are describing it. You could make a version with a color background, save it, and make the content of the second file the thumbnail for the original.
What operating system are you using?
Let me know if you have questions.