what is the whitest material? I saw before a great question on what is the blackest material. Therefore I started wondering, there are many things that look white but in fact absorb light. So what material is truly white? Or, stated in another way, what is the most white material?
 A: "White "
is a body if it reflects all the light shining on it.
Especially it should reflect all wavelengths equally 
"perfect" and the reflection should be diffuse. 
Of course a perfect white surface does not exist. 
Good approximations are dry powders made from high 
refractive index solid particles of optimum 
particle size (Mie theory). 
In practice dry MgO/MgCO3 powder is used as a standard 
to calibrate whiteness meters. This is partly due to 
tradition and partly because it is easily prepared from 
magnesia usta. 
Titanium dioxide powder might be a bit "whiter", if 
powder with optimum particle size is available(?)
Powdered Diamond or glass made with high content of Zr and Hf 
might be the whitest things available.
A: The perceived color "white" occurs either when the source is extremely low or high power, or when there's a balance across various spectral ranges.  Low Power: only your rods respond, and they have no color sense.  High Power: general retinal overload, and no color response.
So, all that's necessary for "white" is a balance of output from the various cones in your retina.  Now, I agree that we perceive white, grey, dark grey, as the total input power decreases, so your question about "truly white" could be translated as "completely reflective at all wavelengths.  WHether the surface then looks "shiny" depends on the smoothness: if very smooth, it's a mirror; if very rough, it's a Lambertian surface which scatters light in all directions (think opal glass).  
In the real world, all materials have some small (at least) absorption at various wavelengths, so the best we can do is either get close to "pure white" or build a specialized material which is perfectly reflective at a specified wavelength (interferometric effects).
