One of the most important and integral instrument of the game of cricket is the ball. The ODI and T20 format is played with the white ball, the Test format is played with the red ball. But none of these are eligible for day-night Test matches.
The white ball gets old too quick and lacks longevity. So, even though it can be seen well in the night when the lights come on, it cannot be used.
The red ball on the other hand lasts much longer than the white ball. But it cannot be seen properly under the lights.
So, an alternative option