To change the material click on the asset you want to use, and go to the materials browser on the the entity components toolbar. This appears in the top right of the screen any time you click any entity. The materials browser looks like a pencil hovering over a little drawing.
Some objects only have one material slot, while others have multiple. Simply click each slot in turn and select from the available materials to assign it to change the colour and texture of that specific area.
Like the asset browser, you can speed up searches by typing specific texture types like 'wood' or 'metal,' or colours to bring up all the red materials, for instance, in different textures.
All materials are available for all entities - so you can stick with realistic textures for your world, or something completely stylised, like a world of robot metal cats. It's all up to you!