How to grow avocado from seed - root turning brown

I have this avocado seed which had split open and roots have developed. However, the main root has turned brown recently. It is not the white root with crystals that I have seen for all my avocado seedlings. This one is different. The root has turned brown and the bottom area is dark brown or black. This is a bit strange as if it is spoilt or dying, the colour of the water will turn brown also. The water is still clear and clean. I notice that one little tiny root has emerged from the side. I hope the seed can survive. I'm not sure how to revive it if it is failing. I will try to change the water regularly to see whether this will help its growth. I have placed it near to the window sill so that the roots can get some sunlight to grow well. Anyone out there had experienced this before and would like to share? I will post updates on this.

Avocado seedling main root turned brown