As far as I know there are 5 receptors for far-red and red light which are the phytochroms(phyA-phyE) Its all about the ratio between red and far-red light.
Each phytochrom has an inactive(PR) and an active(PFr) conformation. phyA is the only phytochrom which is activated by far-red light, so its active state is PR. (Only if the ratio between red and far-red light is low.) The other phytochroms, are activated by red light (high ratio between red and far-red).
An active phytochrom blocks the COP1/SPA complex. This complex is a E3 ubiquitin ligase which ubiquinates transcription factors for the light answer like HY5 or HFR1.
Under normal light conditions, phyB-phyE is active. They block the COP1/SPA complex so the transcription factors for the light answer are not getting ubiquinated. The plant can get a light phenotyp.
In the case that a plant grows under another plant, it gets less red light because the higher growing plant uses it for its photosynthesis. phyB-phyE become inactive. COP1/SPA can ubiquinate the transcriptions factors. The plant gets an low light phenotyp by trying to grow out of the shadow.
The function of phyA is to produce a light phenotyp if there is a lot of far-red light but almost no red light. Than it is getting activated and blocks the COP1/SPA complex. Under red light phyA is not only inactivated it also gets degraded.
I just found a paper for further reading: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2828699/