It is insofar as these objects are coloured and thus visible. It is through the medium of colour that sight is able to attain other objects. Thus, colour is the immediate object of sight and number, movement, rest etc are mediate objects of sight. Whatever sight attains it is going to be in terms of its immediate and proper object, yet is not restricted to this object. The proper object, also called the formal object, is always in proportion to the power of knowledge.
Colour is in proportion to sight.
Let us consider it. It is a spiritual power but operating dependently on a corporeal subject. The intellect, though an immaterial power, depends in this life upon the senses of the body it is in to provide objects of knowledge for it to understand. This is an objective and extrinsic dependence. Thus, the nature of the intellect is spiritual but its natural state is that of objective dependence on the body.