Open the Shortcuts app
Tap the plus icon to create a new shortcut
Tap Add Action
Search for and select Open App
Tap App and choose the app you want to customize
Tap the share icon or the settings icon at the top
Tap Add to Home Screen
Tap the app icon next to the name
Choose Take Photo, Choose Photo, or Choose File
Select the image you want to use as the new logo
Tap Add
Tap Done
Repeat for each app you want to customize
Hide the original app from the Home Screen if desired
Keep the original app in the App Library so the shortcut can still open it