How can I interpret the colour of flames in terms of spectral series? I saw different colors of fire burning different metals. But how does this work? The Balmer series requires electrons to drop back down to n = 2. But, not all alkali metals have empty space in their second principle energy level. Does one electron, from the second principle energy level, just jump to somewhere of energy and come back down. 

My description is kinda confusing, but all I'm  saying is, where does that space in the second energy level come from?
The Balmer series relates to the electronic transitions in the hydrogen spectrum. In other elements it only requires that the energy difference between different excited states corresponds to a wavelength in the visible spectrum. These will clearly differ depending on the effective nuclear charge of the elemnt concerned.