Click on one of the color snippets before placing a fret-marker, a line or a rectangle on the document to use the selected color.
Click on one of the shape previews before placing a fret-marker on a diagram to use the selected shape.
When you click on a color or a shape, you'll see that in the layers panel, the icon and the color will change in the preview. These are the shape and color assigned to the layer.
Every layer will remember the last settings you used so that you can move between layers and always use the same fret-marker for each one without having to select shapes and colors again and again.
You can modify the color ad shape of existing fret-markers using the Select function.
This color only applies to fret markers. The 'double color snippet' will always show the two latest colors you clicked on. So if you want to place a fret-marker that is yellow and blue you have to click on the yellow and blue snippet in the pallette, then the 'double color snippet' will be half blue and half yellow. You can now select it and place your bicolor fret-markers.
Double colors will be saved as 'layer assigned color' as well.