flame test...

Simple. The "lilac" flame given by potassium is not purely lilac / violet. It's a mixture of different wavelengths, which mix up to give you the perception of "lilac".

Notice that with a blue glass, it's NOT the blue light being filtered; it's its complementary color, i.e. yellow, being filtered. Light with different colors will be affected in lesser extent, such as red, blue and violet in this case. Red & Blue mixes to give you a perception of lilac.