Select the layer or the layers for which to change the color and open the color chooser panel in the layer panel to the right. If the color chooser is not visible, select the small check box in the "Color" header bar. Then the color chooser is expanded.
To change the color, click on the desired color. To select a color not offered in the list, select the "More Colors" button. A color choose dialog will open.
To choose the color of the frame to draw around the shapes, without changing the fill color, use the "Frame Color" chooser panel.
Layers can be "dimmed" by making their color darker or brighter so they contrast less with the background. To do so, choose "Dark" or "Bright" from the color panel. Pressing the button multiple times makes the colors darker or brighter each time. The brightness or darkness can be reset with the "Reset" button.