Why my objects are showing in this color?
The image from the tutorial has a white material applied to it. You can see it in the object manager, it’s the white sphere. By default, C4D uses a grey material.  To do the same, create a new material in the material manager (the empty area under the timeline) by using the "Create" menu ->New Material or ->New PBR Material, depending on your preference.  Both of those are white by default.  Then, drag the newly created material to your object, either by dragging to the object in the viewport, or in the object manager.