First create a custom event in your Widget Blueprint, call it ‘Colour Changed’ or somthing, give it an Input variable as an Integer. Then in the blueprint you have shown in the attached image, after each of the set material nodes you have shown, you need to cast to your widget blueprint, get a reference of it somehow (Get all widgets of Class would work), then after the cast node, Look for the ‘Colour Changed’ event, it should let you set the integer value straight away in the node, (use something like 1 for white, 2 for brown, 3 for black).
Now back in your widget blueprint hook up the event node to some logic to check the integer number and fire off event. After I typed this another way would be using ENUMs, but this is faster to setup as what you want is pretty simple.
If u need me to make an example graph screenshot let me know