Good question.   Could be a number of reason, but my guess would be that you have some salt in your boiling water.    When NaCl solution is placed in a flame the Na cation gets ionized and yellow light is given off.    It doesn't take much salt for you to see this effect.   You will see this even with regular tap water, since it also has a small amount of salts dissolved in it.    \nIn fact, standard distilled water is usually not pure enough to avoid this effect.    That's why chemist buy special stills to make ultrapure water for experiments.
Why does a Kitchen gas burner glow yellow when a pot of boiling water overflows?