Any gas should give out only certain colours of light. Then how do CFLs( Compact Fluorescent Lamps)have white color. Do CFLs contain a mixture of gases?
There are two pertinent effects here, as follows.
First, as pointed out by @ofcourseit'snotme, is that the "white" CFL tube does not contain a mixture of gases- it is coated with a mixture of phosphors, each of which radiates a different color of light when excited by the UV inside the tube. Artful blending of these phosphors can yield a variety of resultant colors- the so-called "cool white" (blue tint), "warm white" (red tint) and so on.
The second effect is metamerism, which is the mechanism built into your vision system that interprets an equal mixture of the primaries (red, green and blue) not as discrete sparkles of those colors but as white light.