Pick a level and the electron moves there. A drop sends out one photon, and its color lands on the spectrum below. The four visible hydrogen lines all end on n = 2.
Every drop you make leaves its line on this strip. Jumps that land outside the visible band are counted at the ends.
The electron sits on fixed rungs, so only fixed gaps are possible. Each gap is one color. That is why a glowing gas gives sharp lines instead of a smooth rainbow.