It is a small website/tab icon which can be found in your browser tab when visiting a page. See examples below of some popular favicons.
You can either google hex code for the colour Black or Blue or if there is a specific colour you had in mind you can download a chrome extension called ColorZilla which allows you to select a colour on a website or image and it will give you the hex code.
JPG, JPEG, PNG, GIF, WEBP
Go to Settings > Branding > Preview in new tab > you should see it in your browser URL. It should look like this https://tracking.tryhutch.co.uk/my-store
but instead of my-store
it would be your brand name.