Colour coding lets electricians identify each conductor's role safely. Under the older Indian convention, red, yellow and blue are the three phases (live), black is neutral, and green is earth. Under the newer IEC-aligned scheme you may instead see brown (live), light blue (neutral) and green-yellow (earth).
The single most important rule: the green (or green-yellow) wire is always earth and must never be used as a live or neutral conductor. Mixing up colours is dangerous, which is why wiring should follow a consistent scheme throughout the home.
Always treat any wire as live until proven otherwise, and leave connections to a licensed electrician. APAR house wires are available in the standard insulation colours to support correct, code-compliant wiring.