Because of the amount of light striking each face. The foamboard facing the front (south) is getting full sunlight while the foamboard on the side is getting less. Now, I could have exposed the photograph so the frontal foamboard appeared its proper pink, what would have happened to the west foamboard? It would have been darker.
The amount of light captured by my camera renders the front foambard more whitish than reality but captures the correct color on the side. In fact, the less light the more vivid the colors tend to be (of course too little light yields black). Study how to vary the exposure on your digital camera and experiment, with digital cameras such experimentation is cheap.
Digital photography is a blessing as you can verify before you leave the scene your exposure is what you want. However, in practice most people just point and shoot and as a result their photos tend to be just as poorly exposed as they were with traditional film photography.