The Paint tool is helpful in several ways. It's a useful communication tool that allows you to make marks on scans or other meshes. This can be a helpful reminder of a desired goal or outcome to yourself or another practitioners. Note: these colors will not be printed when sent to fabricators and are only utilized during the design phase. 

Select the Paint tool on the left menu and a color of your choice. 

It can also be a helpful tool to fix coloration issues. View the below clip and notice the ridge on the distal end of the scan. Select the Paint tool and toggle on the "Color Picker" option. Hover over the the color that you would like to select- the Color Picker selects the color under your mouse. Next, paint over your desired section.