New Button Slicer (November 2023 Update)- Conditional Formatting

You can try this easy workarounds:

1.for different colors when button is selected - create  shape with desired color for selected value, place on top of desired button,format-> send to back , then formating->change background to 100% transparency and also callout background of selected to 100% transparency. This way on button select you ll have the color from the shape underneath.

2.If you need different colors on selected and on default: create text box to mimic the button (font, size)  and the desired color , put it on top of button slicer ,then ,  make a bookmark to show hide text object; next and add a button on top of  slicer button same size , set it to 100% transparency , no borders and assign the bookmark. This way you have an aditional color (on default state) but you loose color change when hovering over ... 

At least like this you have some solution until maybe this is fixed and your visual is still based on slicers .