Okay, after a quick bit of playing around, the only way I can get one device to work is if I touch the two legs of the Phototransistor together, which changes the RGB to blue, then if I let the legs come apart again, the RGB changes back to red.
I'm not quite sure what this means, however. Is the signal too strong for the Phototransistor? Or, do I need to find a matched pair of Phototransistor and Emitters and fit them in opposite circuits?
Bear in mind this is only a prototype and I'll be happy if only two devices work against each other
EDIT: Would something like 2 sets of these do? http://www.rapidonline.com/productinfo.aspx?tier1URL=Electronic-Components&tier2URL=Optoelectronics&tier3URL=Infrared-Devices&tier4URL=Matched-T1-infrared-emitter-and-phototransistor-pair&moduleno=29405
EDIT 2: Okay, so to test something out I removed the Emitter from one of the devices (the other device was switched off) and the Phototransistor was still detecting IR light, even when in a dark, secluded room. After this, and to ensure that the red light was not on constantly and was actually working using the IR light detection, I removed the Phototransistor from the circuit. After removal of both of the components from one device, the RGB LED did indeed glow blue. This leads me to believe that there is either a fault with the Phototransistor itself or with the detection code. Any ideas?