We cannot control the color, but we can change the image in each speeddial. I suppose that you know this; but I want to give a note to new Opera users here.
How to use site logos in Opera Speeddial?
When you open a speeddial click to heart icon at the right end of the address bar then a pop-up windows will be opened. At this window, we can select one from many different images on the web page. Most probably one of them is the site logo. You can select and set it as the speeddial image for the site.
Here is an example:
Here is my startpage:
ps. In order to remove titles of speeddial delete its name!
@grgnd thank you for the support. Adguard filter support would indeed not only improve the built-in adblocker, but also make more people migrate from adblocking extensions. A win-win for Opera and the users imo.