Explain (in words) why Li produces spectral lines and why it appears pink in the firework?

Color tells us about the temperature of a candle flame. The inner core of the candle flame is light blue, with a temperature of around 1670 K (1400 °C). That is the hottest part of the flame. The color inside the flame becomes yellow, orange, and finally red. The further you reach from the center of the flame, the lower the temperature will be. The red portion is around 1070 K (800 °C).

The orange, yellow, and red colors in a flame do not relate only to color temperature. Gas excitations also play a major role in flame color. One of the major constituents in a burning flame is soot, which has a complex and diverse composition of carbon compounds. The variety of these compounds creates a practically continuous range of possible quantum states to which electrons can be excited. The color of light emitted depends on the energy emitted by each electron returning to its original state.

Within the flame, regions of particles with similar energy transitions will create a seemingly continuous band of color. For example, the red region of the flame contains a high proportion of particles with a difference in quantum state energies that corresponds to the red range of the visible light spectrum.