Red aquatic plants
Red plants are the greatest challenge in planted freshwater aquariums. They require bright light, iron, CO2 and often low nitrates.
Detailed explanation
The red colour is given by anthocyanins produced as protection from intense light. By difficulty: Alternanthera (easy), Ludwigia palustris (easy), Rotala rotundifolia (medium), R. macrandra (difficult).
You need light 100+ PAR, CO2 at 30 ppm, iron 0.1+ mg/l, moderate nitrates. Iron is the most critical nutrient for reds.