Question

A point source emits light of only a single frequency = 6.7 x 1014 Hz. What color is the light produced?

Answer 1

It is easier to figure out what color it is by the wavelength, so find it:

$$\lambda=\frac{c}{f}=\frac{3\cdot10^8}{6.7\cdot10^{14}}=448\text{ nm}.$$

448 nm is the wavelength of blue color.