Does light frequency change?
No not really
When light is created, like in fusion or when an object is heated, It has a fixed frequency that is the same until it's absorbed.
When light is created in the sun, it comes out as a photon consisting of all the colours. When this photon hits an object some of it's colour gets absorbed and the photon continiues with less colour than before. If the object is seen as green, all the colours exept for green is absorbed and the photon continiues with only green colour left and when it gets absorbed by our eye and making us see the object as green.
Colour is just different frequencies of light that we observe as colours. So light from the sun consist of all the different frequencies and different objects absorb different frequencies of light.
So to summarize, light does not change frequency but the different frequencies inside of the photons gets absorbed and therefore leaving the photons with other frequencies of light that we absorb and perceive as colour.