A lighting system includes a housing, a light-transmitting top member positioned across an open top of the housing, a light source positioned in the housing, a helical fiber biased between the light source and the top member, and an adjustment means for adjusting the compression of the helical fiber. The helical fiber is light-transmitting and is doped with a first wavelength-converting material. A portion of the light emitted by the light source passes through open space between turns of the helical fiber, and a portion of the light emitted by the light source is received by the helical fiber and converted into a light of a predetermined color. Adjusting the compression of the helical fiber changes the relative portions of the light that passes through the open space between the turns of the helical fiber and the light received by the helical fiber.