Hi max..

Usually red sems are a sign of a nutrient deficiency which will be accomponaied by stunted growth and woody scrawny stems..

There is no way to 100% be sure of what the problem is until you can confirm that you have been watering with water that has a PH of 6.5 - 7.0.

From the pictures, It does look like your plant is Nitrogen deficient,

iloveyou