Step 1: Link size and light:
Visible light is about 400-700 nm, and colloidal particles are of similar size, so they scatter it.
Step 2: Colour from scattering:
What reaches the eye is the scattered or transmitted light, so the sol's colour comes from this scattering, known as the Tyndall effect.
Step 3: Other properties:
Brownian motion, electrophoresis and electroosmosis deal with movement of particles or medium, so option (A) is the one tied to light and colour.
Final Answer:
The Tyndall effect gives colloids their colour.
\[ \boxed{\text{(A) }\text{Tyndall effect}} \]