Phosphates and iron will also play a role, but not nearly as much as nitrates. Lower phosphates and higher iron will help create a blood-red color. Ironically, the red color in these plants is actually a symptom of nutrient deficiency...kinda like being intoxicated with alcohol is actually a symptom of alcohol poisoning. Anyhow, keep the phosphates low, .1-.5ppm, the iron high(er) and you should have red results.
I grow both of these plants (R. Indica & L. Repens), and I always know when my nitrates are getting low because they will turn red. They don't turn a deep maroon because I keep high phosphate levels, about 1.0ppm.