Hello, buidiena3.

Sorry, you are probably not going to like this answer.

The reason why all of the icons in your toolbar are not black and white is that, maybe six to eight or so years ago, the designers at Apple decided that you would like it better that way, and instructed app developers such as us to remove the color from our icons, which we did.  In BookMacster's Preferences > Appearance, you can set it to a "grayscale" icon (3 smiling bookmarks), but, sorry, no color.  You must have updated from a very old version of BookMacster, because we did this several years ago.

Apparently, based on your report that it worked until the latest BookMacster update, color icons still work.  However, from past experience app developers know that when Apple instructs us to do something, we can get away with not doing it for a while, but eventually our app will be broken by a future macOS update.  So, we have no plans to make the color icon available again.

The design direction of Apple from years 2011 to 2018 has been to reduce contrast, making things pastel, hard to find and hard to read.  This was reportedly due to the direction of a certain Apple vice president, Sir Jonny Ives, who departed Apple a year or so ago.  It is widely believed that Jonny may also be the one responsible for making Mac laptops so thin that the keyboards became unreliable, and in a recent MacBook Pro redesign, Apple reverted to a thicker keyboard.  So maybe your color icons will come back someday!

Personally, I just wish that Apple would put back the title bars back on windows.  As part of this same design trend of 2011 to 2018, window title bars were removed in a macOS update a few years ago.  Or at least Apple could give me a dime every time I grabbed the wrong window due to the lack of title bars.