Why might scientists mainly astronomers study the color of metals in a flame?

Answer 1

The colours with which an element (metal or other) burns in a flame are the same colours which are absorbed by that element in the sun's spectrum. The colours are very specific to each element and show up as distinct black lines in the solar spectrum. By studying the spectrum it is possible to tell what elements are in the sun [or star] and also their relative abundance.

If a star is moving away from us absorption lines are red-shifted [move towards the red end of the spectrum]. The faster the star is moving away, the greater the red-shift. Also, as Hubble discovered, the greater the red-shift, the further away the star is. So the red shift in the absorption spectrum is a measure of not only how fast the star is receding from us but also how far it is. This allows us to tell whether two stars which are apparently in the same direction are actually close together or simply a coincidental alignment of their lines of sight.

Answer 2

The color in flame is a characteristic of a metal.