The original intention was to use a fischertechnik light source and two fischertechnik light sensors in digital mode. One sensor measured light transmitted through the platform to detect the presence of a draught; the other measured reflected light to distinguish between black and white. This system did work, but it was tricky to set up and very susceptible to ambient light.
More successful was one of my own reflective IR sensors. These were originally made for use on line following buggies; they are dirt cheap and very simple. They can be used in digital or analogue mode, relying on the project board's pull-down resistor to complete the phototransistor circuit. These are susceptible to ambient light too, but much less so. With the Picaxe input in analogue mode it proved quite easy to distinguish between a white draught, a black draught and the red platform.
Invaluable for checking this out and determining the relative levels was the Picaxe facility for transmitting serial data back to the PC, used with a simple test program.
In the main program the light levels are defined as constants, which makes them easy to change. In the event it did prove necessary to change them when extra lighting was used for filming.
A consequence of using a single light sensor was to free up one of the Picaxe inputs.