My observations contradict this statement. Formerly, I was never able to get Ludwigia arcuata past a dull orange, but now I can (consistently) get reds out of this and other plants by raising the P to N ratio. You can have blood red plants up to even 10 ppm NO3 if you keep PO4 at 1.5-2ppm.The ratio of N to P isn't so important.
Outright stressing the plants with both lean P and N just resulted in dull, slow growing plants. I observed high growth rates and excellent health, however, with high P to N.