Whenever you use the Display title/price on hover configuration, prices are only visible when the customer hovers over them. The image overlay background is the color of the product name and price block that appears on hover. This setting can be enabled on collection pages and on home page featured collections.
By default, Ira uses the Always display title/price configuration, so you won’t see the image overlay background.
You can learn more about how the overlay works on the documentation for the Collection page.
The image overlay text appears on product images if the Display title/price on hover feature is configured. This setting can be enabled on collection pages and on home page featured collections.
There’s an example how this looks with the Image overlay background documentation.
If you haven’t already picked a color palette for your store, you may be interested in these resources that could help you pick an effective scheme.