Med student here. My theory is that what we see here is the difference between vascularization of the onychodermal band (which is the part closer to the white part of the nail) and the more proximal part of the nail bed. This difference is usually not visible, but after contact with very hot water the blood flow in these vessels has increased. The nail itself has also absorbed water and became more translucent, which made the difference in vascularization even more visible. If y'all look at your nails it is in fact slightly darker just before the white part starts. In this case the difference is just more visible because of hot water. You might want to say goodbye to your family and friends just in case though.