All Mari Tips
You can pick a color from an image by loading it into the Colors palette and selecting the color you want. See more at Selecting Colors from a Picker or Image.
You can customize your brush and test it out in the scratch pad at the bottom of the Tool Properties palette. See more at Customizing Your Brush.
Use Shelves to store your favorite brushes, images and colors so you can easily access them across multiple projects in Mari. See more at Storing Resources Using Shelves.
Select and shake a node with your mouse to quickly disconnect it from another node. See more at Connecting Nodes.