Aquariums: Nitrite testing query

Hello everyone,

I started a new 63 gallon tank 2 weeks ago and have been using seachem prime and stability for the quick start process. I've gradually added 23 rummynose tetras last week and they seem healthy and active with bright red noses.

I've been testing the water parameters and they consistently read ammonia 0, nitrate 0. I am wondering how people would interpret this recurring darker blue colour from my nitrite tests. The colour is consistent even after a 40% water change two days ago.

Could it mean that the nitrite reading is between 0-0.25ppm or can it be because if the heavy tannin in the water? Thanks for reading everyone.