To be fair, I don't think The Secret references Kant at all. This is my own interpretation of both works.

But Kant does have a good answer for your apple scenario: Of course the apple is red, and to call it blue means either your senses are faulty or you are willfully disillusioning yourself, to use your words. The Secret wouldn't contest this either; the change that the power of positive thinking brings about is not in the actual properties of things themselves but in their relation to each other in the world around you.

So we say that the apple is red, and redness is a property of the apple. How red is it? Well, this particular apple is just a pale red, almost sickly looking. I would rather a robust redness, wouldn't you? The redness of the apple is brought about by its gestation on the tree, which is a function of causality, and causality follows from time, which we've already discussed is a precondition of sense perception, not a property of the apple itself. So, The Secret might argue (but probably not Kant, I should add) since the causality that brought about the redness of the apple in our perception is all in our head, we may have some power in changing it. Clearly, the redness of this apple is already given by virtue of our having observed it in this time and space. But positive thinking could change the redness of apples in our perceived future; the apples don't change, because they exist outside of time and space, just our perception of those future apples by way of the causality that exists only in our perception. Can you already see from this argument how easy it is to now integrate a discussion of collapsing waveforms and Schrodinger? It's all about as yet unperceived causal chains which aren't part of the things in themselves but depend on our own perception.

On the subject of trashy self-help books, have you read The Power of Now? It was a big Oprah book awhile ago. In it the author mentions Kant not at all, but it is very much rooted in Kantian epistemology. I like it a lot more than The Secret, and I think you might too, if only by virtue of the fact that he doesn't attempt to draft any hard pseudoscience into his arguments.