Before we can answer why drain is white, we have to clarify why anything at all seems white. It is simply an issue of straightforward material science. The motivation behind why things seem white is on account of they mirror every single light wavelength and assimilate none. On the off chance that all wavelengths of light were retained and not were reflected back, the question would seem dark.
While drain is fundamentally made out of water (approx 87%), which is a lackluster fluid; there are different segments of drain (fat, protein, lactose, minerals and vitamins) that impact the shade of drain. The fat and protein particles in the drain reflect light at a wavelength that influences the fluid to seem white.
If we somehow managed to expel a portion of the fat from the drain (like in skim drain), it would give the drain an alternate shading due to the wavelengths the light is reflecting back to our eyes. That is the reason skim drain has a pale blue tinge.