Hi,
I live in Denmark. I keep my Pilosocereus Azureus inside the apartment during the winter months. I bought it in 2020 but it is just in the last 3-4 months that it seems suffering. The head is turning bluish/violet.
What is it happening? What shall I do?

The soil is composed by pumice stone, cactus soil and coarse river sand (1/3 each) together with a bit of small lava rocks.