Well, the results are in (actually they've been in for awhile), and I must say both the boy and I were pleasantly surprised. From the beginning to the end, the results have been dramatic.
As can be seen from this photo, the blue food color stunted the growth of the paperwhites. The difference was a statistically significant 6 cm on average reduction in height between the blue group (on the right) and the control group (on the left). Oddly, the yellow group (in the middle) showed no statistical difference in growth.
The upshot is this: if you want to stunt the growth of paperwhites growing in water so they don't flop over, it's worth giving blue food color a try added to the water. Even if you can't duplicate these results, the paperwhites and their containers will look pretty with vivid blue water.