Because there are no rays of light falling on them; hence, there is no color for them to reflect.
Does an object reflect its color in every direction?
It does reflect its color in every direction.
How do we know this?
Because thousands of persons, in different directions from the same object, can see it at the same time.
How are the rays of color reflected from an object?
They are reflected in straight lines.
Where must a person be so as to see an object?
He must be in a position so that there is nothing to prevent the rays of light passing in a straight line from the object to his eyes.
What color does a red ribbon reflect?