Step 1: Identify the type of system.
Milk dispersed in water is a colloidal system. The fat globules in milk are colloidal particles (size 1-1000 nm).
Step 2: Recall the Tyndall effect.
When light passes through a colloidal system, the colloidal particles scatter the light. This scattering is the Tyndall effect.
Step 3: Explain wavelength dependence.
Scattering intensity is proportional to \( 1/\lambda^4 \). Shorter wavelengths scatter far more strongly than longer ones.
Step 4: Identify which colour scatters most.
Blue light has the shortest wavelength (about 450 nm) among visible colours and is scattered most strongly. Red light (about 700 nm) is scattered least.
Step 5: Apply to reflected light.
When we view the milk-water mixture by reflected light, we see the scattered light. Since blue is scattered most, the mixture appears blue. This is the same reason the sky appears blue.
Step 6: State the answer.
Milk-water mixture viewed in reflected light appears blue. \[ \boxed{\text{Blue}} \]