According to Locke, colors are nothing but powers in objects to produce ideas in our minds. He writes: “What I have said concerning colours and smells, may be understood also of tastes, and sounds, and other the like sensible qualities; which, whatever reality we by mistake, attribute to them, are in truth nothing in the objects themselves, but powers to produce various sensations in us, and depend on those primary qualities, viz. bulk, figure, texture, and motion of parts; as I have said.” (Essay Concerning Human Understanding, Book II, Chapter VIII, section 14) There is, he tells us, no “resemblance” between these sensations or ideas and the objective ground of the power, as there is in the case of primary qualities, and without minds to interact with the powers there would be no color (etc.) in the world. To say that an object is red is just to say that it has the power to produce sensations of red in us: that is, colorless objects made of colorless minute particles happen to cause in us a certain type of sensation, and that is all there is to color—it has no mind-independent existence.
But two questions may be raised about this doctrine, appealing though it is. The first is that a given object has several such powers: it may produce sensations of red in one set of perceivers and sensations of green in another set (and so on for the other colors). It doesn’t have a unique idea-producing power but many such powers (it could even produce ideas like those produced by the other senses). Yet there is only one set of primary qualities underlying this multiplicity of powers. This prevents us from identifying the color with the primary quality basis, on pain of identifying the colors with each other (same basis for each power and hence sameness of color). It isn’t the power in the object that is fixing the color of the object but its relation to the minds of perceivers. Second, how could a colorless object have the power to produce sensations of color? Nothing about the object itself explains the power it has to excite color sensations, let alone specific color sensations. It is, we might say, chromatically impotent. It would be a type of miracle if primary qualities had the power to produce sensations of quite different qualities. The only sense in which objects have such powers is that minds have corresponding powers: the mind has the power, when interacting with colorless objects, to generate sensations of color—the power comes from it not the external object as such. The power of which Locke speaks is really a relational power not an intrinsic power. Intrinsically the object is powerless to produce color sensations; whatever power it possesses is conferred on it by perceiving minds. Locke should have said that colors consist in the power of our mind to impose colors on the world—powers in the mind not powers in objects. Does a square object have the power to produce sensations of a round object? Well, it can produce such a sensation if the perceptual system misfires, but it has no intrinsic power to do any such thing—as it has to produce a sensation of a square object. It just happens to cause (partially) a sensation of roundness; the real work is done by the perceiving mind—it has the power to respond with roundness perceptions to a square object. Similarly, colorless objects can elicit sensations of color, but only because minds are so set up that they can generate sensations of color in the presence of things that have no color. The external primary qualities play a minimal causal role, and considered in themselves have no power to produce sensations at all. Indeed, it is conceivable that there be no such objects and yet the mind has the power to generate the full panoply of colors from within its own resources (color sensations in a vat). In fact, we are already in a situation close to this in that we have colored mental imagery that is elicited by no external object—no primary qualities are triggering this kind of “perception”. The external object in the perceptual case merely triggers a pre-existing power of the mind; it is not the locus of the power to bring color sensations into the world.
If we say that water has the power to dissolve salt, we mean that water has objective properties that explain how the power is exercised; but if we say objects have the power to cause color sensations, we can’t provide any such explanation. This is because objects are powerless in this regard; the mind is the origin of the power in question. Imagine a world in which there are simple objects having just two properties and yet these objects are perceived by minds as being rich and complex, endowed with (say) a thousand properties. It would be bizarre to suggest that the objects have the power to produce the full range of sensations available in this world—that power properly resides in the minds that exist in it. It would be quite wrong to say that the properties perceived are nothing but powers in the objects to produce sensations, with their impoverished two-property profiles. The objects have no such intrinsic power, though they have the weak relational power of being able to interact with minds that do have the power to perceive the full range of a thousand properties. In a way Locke undersells his own subjectivist position, which is that the mind is the origin of secondary qualities not the external world. At the least he should have distinguished between the weak sense of power and the strong sense, maintaining only that objects have only a weak power to cause sensations. Primary qualities, by contrast, have a strong power to produce sensations because of that “resemblance” he mentions, but secondary qualities are only weakly connected to the objective nature of external objects. They are connected in roughly the sense in which sugar has the power to taste bitter if the taster’s sense organs are deranged in some way, or in which red objects have the power to appear yellow if there is something wrong with your eyes. The relation expressed by “x can trigger y” is much weaker than that expressed by “x has the power to y”.
This matters because it affects the strength of subjectivity of Locke’s basic doctrine. If we identify colors with powers-in-objects, then we accord them a degree of objectivity not intended by the basic metaphysics; but if instead we identify colors with powers-in-the-mind, then we fully endorse the fundamental thesis that colors are purely subjective, i.e. originate in the mind and are then imposed or projected onto external things. Here is Locke in full flight: “The particular bulk, number, figure, and motion of the parts of fire, or snow, are really in them, whether anyone’s senses perceive them or no: and therefore they may be called real qualities, because they really exist in those bodies. But light, heat, whiteness, or coldness, are no more really in them, than sickness or pain is in manna. Take away the sensation of them; let not the eyes see light, or colors, nor the ears hear sounds; let the palate not taste, nor the nose smell, and all colours, tastes, odours, and sounds, as they are such particular ideas, vanish and cease, and are reduced to their causes, i.e. bulk, figure, and motion of parts.” (ibid, section 17). That use of “reduced” is perhaps ill advised, suggesting as it does that secondary qualities, and ideas of them, are reducible to primary qualities of external things, which would make them as real as primary qualities in general. But it is clear that Locke intends to maintain that colors (etc.) are mere subjective projections not at all inseparable from matter—they arise from powers of the mind not from powers of matter. It is only in a very weak sense that we can say that objects have the power to produce ideas of secondary qualities, as weak as saying that manna has the power to produce sickness and pain, or a spear has the power to be thrown. The truth is (according to Locke) that colorless particles interact with our colorless sense organs in such a way as to activate the latent power of the mind to generate colors in all their glory. The power to produce color sensations is a mental power not a power of material objects considered in their own right. Perhaps Locke was subliminally influenced by his opposition to innate ideas: for if colors originate in the mind, how can ideas of them be derived from perception of external objects? At any rate, one who sympathizes with Locke’s metaphysics of color has reason to dislike his official object-centered formulation of the doctrine. The powers that give rise to colors are in the mind not in the external world.
 Much the same can be said about the dispositional formulation of color subjectivism: objects have many color dispositions and no object has such a disposition intrinsically. Rather, minds have dispositions to see the world as manifesting various colors—that is where the disposition originates. Objects are merely disposed to trigger such mental dispositions in the sense that they can trigger them in certain conditions. The ontological work is done by the mind not the world.
 None of this is to deny that matter might be the origin of the mind’s power to produce colors; it is just that the material objects of perception are not the locus of the power to bring colors into being. Put broadly, Locke (and his followers) are putting too much emphasis on the powers of objects and neglecting the vital contribution made by the mind.