Muladhara chakra is the lowest, but most important of all the chakras. Like it’s name, which literally means the “foundation root”, any issues with it reflect in other chakras. In Hath yoga traditionally this was the first chakra that was worked upon, same as while building the foundation is the first one made.
It is visually shown as a Red Square. I have used Sindur (vermillion) pigment mixed with linseed oil. The muladhar is also the seat of the unconscious. Within the square is what I imagine as the unconscious.