Before taking this part of the course I never really thought about what makes a colour or how to change it, I was going with what I felt right instead of conscious choices. I gained better understanding reading Basic Colour Theory by OCA, Michael Freeman’s book  and finding couple pages interesting on digital-photography-school.com   as well.
So what makes a colour? Hue, saturation and brightness. To be precise, hue is the colour which can be changed by using filters or altering the white balance setting on camera. Saturation is the intensity of the hue which can only be adjusted slightly in camera (in my Canon camera this option called Picture style), or in post processing for greater control over the amount of grey in a colour. Brightness is the lightness or darkness of a particular hue, the amount of black and white found in a certain colour which can be changed easily in camera by varying exposure.
Changing the exposure can be done by altering the aperture, shutter speed or ISO, referred to the exposure triangle, or applying exposure compensation on a DSLR which effectively changes the shutter speed (Aperture priority mode) or the aperture (Shutter speed priority mode and Manual mode). For this exercise I needed to choose the aperture as a variable.
The first one I took to define the closest match to the actual door colour was the third image with F/4, all images were shot at 1/125 shutter speed and ISO 320. I left the white balance on auto, I shot these quite late afternoon in the shade. Lining up these images clearly shows how much the colour changes just by under or overexposing an image, at F/2 it’s very nice light blue, at F/8 it’s almost black, and also increases or decreases the highlights as well.
Although I’m sure I have been shooting like this all the time finding the right exposure, I never thought of this as a tool for creating different colour effects, normally I like to change the exposure to the brighter side rather than dark, however this exercise made me create some darker, moody images too for my third assignment.
 Michael Freeman – The photographer’s eye, Chapter 4: Composing with light and color, Page 109