I am really fascinated by colour. In this talk by Beau Lotto he shows a lot of optical illusions to demonstrate that our perception of colour is dictated by context and not by the actual wavelength itself. As photographers I believe we are also working with optical illusions. Our perception of an image at the point of capture is affected by the context of our surroundings, both inside the frame and outside. The camera, on the other hand, simply sees the wavelength of the light. So, we may be affected by the fact that we know it is Winter, our hands and feet are freezing and so the tonal quality we expect in our image is towards the cooler blue end of the light spectrum. In processing the image the camera 'saw' we often have to work to restore the 'white balance' to our original perception of the in-context image. We sometimes even warm up or cool down an image to strengthen the context and add impact to the visual experience.