I am not sure that keeping NO3 at 5ppm will do it as far as making plants redder. The ratio of N to P isn't so important. The question to ask is, will the plants actually be nitrogen deficient at a NO3 level of 5ppm. I suspect not. I got some nice red plants back in August in a tank where the NO3 level was not measurable. I got some Hygrophila polysperma to produce a nice pink-red color and some Blyxa japonica also to turn pretty much red, with red striping on the leaves and pale green inbetween. The plants were still growing, but not as fast as they could. Interestingly, the plant that suffered with actual N deficiency and die-back of some of the older leaves was Zosterella dubia. Plants experience low nitrogen levels frequently in Nature, and are not harmed much, unless the deficiency is really extreme.