I transform the wavelengths of light everyday in the laboratory; so do you if you have a green laser pointer. For how a green laser pointer works, see http://www.repairfaq.org/sam/laserpic/glpdpics.htm
This is called non-linear optics, and requires (a) higher than normal intensity of light, and (b) a properly cut crystal that meets specific structural requirements.
So if you want to convert yellow light to green light you will have to work harder, because the simplest processes change the wavelength by a factor of 2: double it, generating two new photons from each one destroyed, or divide it in half, which requires two photons to be destroyed while creating one replacement with double the energy (half the wavelength).
This was first demonstrated by Peter Franken, et al, at the University of Michigan, in 1961. The paper is discussed here, with references: http://www.laserfocusworld.com/articles/2011/09/sprc-symposium-2011.html