Hi Peter...Welcome to the forum
To answer your question we need to know what the plant is.
What is your substrate?
Is the tank Cycled?
pH, KH, GH, Nitrate, PO4
How much light, what kind, and how long each day
How heavily planted is the tank?
CO2 injection?
Chemical Fertilizers added?
Fish Load?
Otherwise it would be just a guess why it is that color.
Regards,
Jay