Icons in Safari's pinned tabs

Pinned tabs are very useful feature in Safari and I'm using it all the time.
But, I'm having some experience problem with it.
When I pin several tabs from the same domain but different subdomains, I'm getting the first letter as the icon of this pinned tab to be exactly the same. Then it takes me a while to find the tab I was looking for by switching through them one by one.

Is it possible to manually change the icon for these tabs or at least make it to use the first letter of the subdomain instead of the domain name, so it will be much faster accessible?

Solution 1:

There is another workaround: Add the webpage of your pinned tab to your bookmarks. The pinned tab will then show the first letter of your bookmark title. So this way you can at least make sure that all your pinned tabs have different letters, which might already be sufficient :D

Solution 2:

The Icons you see are coming from the web server. If Safari does not find a favicon then it makes one up using, you guessed it, the first letter of the domain.

On the very off chance that you have access to the web server you can upload a unique favicon to each subdomain. Otherwise the answer that Tetsujin has provided (using submenus in the favorites bar) may be the closest you are going to come.