If all you want to do is change filament at a given layer.
-
add your filaments under the filament section in preview
Click on the number to the left of the filament to set the color of the filament. -
slice your model
-
move the layer slider to find the layer that you want to change at.
You can see that I have the slider on the right set to 5.2mm. One thing to note is that this is really the layer that starts at 5mm and ends at 5.2. -
right click on the plus sign on the slider and select change filament
(I selected blue for this layer change.) -
re-slice the model and you will see the color change incorporated into the preview.
Note: if you are just changing filament for full layers this way then you can turn off the prime tower.