Which materials redshift light? I'm currently researching ways to get the human perceived colors red, green and blue when you only have a mostly blue spectrum available at your source (using electroluminescence). The solution should be quite simple so that an engineer can build this without needing a laboratory.
Simple color filters filter every frequency but one frequency range, so the light gets darker. So this is not very feasible to use this.
However then I found this quote on wikipedia:

high luminance inorganic blue phosphor is used in combination with
  special color conversion materials

So there seem to exist materials which red shift blue light by passing the light though (?) these materials. Do such materials exist?
According to this answer (Is it possible for a material to shift the frequency of all light reflected off of it by a specific and constant value)
only one technique was named (Raman scattering) which is again not practical feasible to use.
 A: There is likely no practical and straightforward way to do what you want.
An acousto-optic modulator can shift an optical frequency by a fixed amount, but the amount of the shift is small, not big enough to cause a difference of color perception. It is also not particularly efficient; typically a few percent of the energy in the input beam is converted to the new wavelength and the majority passes through without changing.
A phosphor can generate a large color shift, but the output color is basically fixed; it depends on the band or orbital structure of the phosphor chemical, not  on the color of the input (charging or pumping) light.
A: you are basically looking for fluorescent materials. These materials are coated inside the fluorescent lamps which absorb the ultraviolet light and convert it into visible line radiations. Rare earth compounds such as Gadolinium oxy sulphate are (were ?) very commonly used phosphors. However new research is going on you may find this article interesting. 
The process of color conversion inside the phosphor can be understood as, when a high energy photon (blue or UV) falls on the phosphor the atoms inside the phosphor excites into higher energy levels. These atoms are then de-excited through several routes and emit photons of smaller energy. 
This is not exactly frequency shift of light but rather absorption and emission.
I hope this will help,
