This page of the application settings dialog lets you manage the program's presets for particle colors and sizes.
Default particle colors and sizes
When importing a new simulation file, OVITO automatically assigns colors and radii to particle types based on their name, in particular chemical atom types such as "He", "Fe" or "Si". The table shows the predefined mapping of particle types to default colors and display radii. You can edit the colors and radius values if desired and OVITO will remember these default presets across program sessions.
Changing these presets is also possible from the particle types panel, where you can change the appearance of individual particle types after importing a file. The from that panel updates the color and radius presets in this table and also allows to define new presets for particle types other than the chemical elements already listed here.