Apple - Time Machine Icon on Desktop showing the yellow icon

To fix this I went into Get Info for the external drive and under the "Preview:" section it showed the TM icon, which I clicked and dragged to the top of Get Info and it overlayed the yellow icon.

For me it fixed by following these steps:

• Go to System preferences > Time Machine
• Go to Select Disk...
• Select the disk which is showing yellow icon
• Start the backup

This will remark the disk as Time Machine Backup and macOS will change the icon to the Time Machine green icon.