The flame colour produced by potassium compounds is

Explanation

Heating excites the outer electron to a higher level and the colour is emitted as it falls back, the energy gap being characteristic of the element, so potassium gives lilac, sodium golden yellow, calcium brick red and barium apple green. The flame test is the simplest analytical use of atomic emission spectra. Sodium contamination easily masks the potassium colour, so it is viewed through blue glass.