Lightning : Icon color is not changing using the fill CSS

Actually the issue is which element you are applying the "fill" property to. Meaning when you declare something like . It gets rendered as follows HTML Render If you notice the "icn" is applied to the outer span container rather than svg itself. Hence you will have to do Something as follows to apply your custom styles. ( applying the css to the svg element using heirarchies )Svg Icon Colored Updated for LWC components. (18/03/2021)The above mentioned approach would not work for lightning web components such as lightning-icon as the web components use a shadow root property "closed" - which prevents us from modifying styles or accessing attributes from different namespaces. Luckily Salesforce has introduced Styling Hooks for us to customize the icon styles. Add a class attribute to the lightning-icon component. And add the following properties to your css class to customize icons.

.comments-icn{
   --sds-c-icon-color-foreground-default: orange; //utility icons
   --sds-c-icon-color-foreground: orange; //action icons or custom icons
   --sds-c-icon-color-background: white; //action icons or custom icons
}

It Could be a release related thing (some update to the DOM structure) or if there is a shadow root intervention for the out of box components - then this approach might cause discrepancies as this relies on the DOM heirarchy & CSS Specificity. Nevertheless please feel free to update your approach with an example for the future audience. This way you can change the lightning icon