The quick tool icons were changed after Acrobat X during the overhaul of Acrobat's UI to DC and beyond. One of them was to make the tools in the quick access all GRAY, and eliminate their COLOR unil the pointer hovers over it.

This was a terrible decision and backwards step from a usability standpoint. Humans interact in their environment by perceiving things in color, lots of colors, and rely on color to understand and navigate the world. While shape outlines are one point of information to understand what a thing is, color adds considerable information to that mental processing and recognition step (read: speed and thus efficiency). Eliminating color thus slows visual acquisition of the desired object/target/tool. This all seems like some ill concieved desire to make the UI look more uniform and "pretty" but that came at the considerable expense of usaiblity. While that might be fine for the masses, completely eliminating the ability for a user to enable "always in color" as an option for the icon set is a gross dereliction of duty for the UX team or executives who signed off on this "new" and "better" product.

This is not better for some Pro users who have customized the toolbar with many tools. More tools means a greater need to visually distinguish from among them. Here's one post from a number of disgrunteld "customers" (assuming Adobe even thinks of us that way anymore) https://community.adobe.com/t5/acrobat-discussions/acrobat-dc-two-rows-of-toolbars/td-p/12953009

On a separate but related note, I also see some icons have been simplified, also obscuring the critical information about WHAT tool it is: Three different tools now all have the same icon, which is not what it was in Pro X.

Acrobat needs to understand that Acrobat Pro users are not Acrobat Reader users, and the UI or software itself should reflect those separate markets. Acrobat needs to think of Pro users more like developers/coders who dislike the dumbed-down button-based UI and ensure (1) screen real estate is carefully evaluated (2) keyboard navigation and use is maximized, and (3) customization options are prevalent for each.