Changes to the wiring colour codes mean that fixed electrical and mains-powered cables (subsequent to the introduction of new cables) will feature the same colour wires as any flexible cabling. The blue wire also referred to as the neutral wire, has the function of transferring electricity away from the appliance. The brown wire, otherwise known as the live wire, transfers electricity to the appliance. The combination of these wires is referred to as a circuit. You should be aware that some properties have old-style wiring, which should be regularly checked by an electrician to ensure that it remains in good working order. The need for replacement will be entirely dependent on the safety of the wiring.
The green and yellow wire is also referred to as the earth wire and has a key safety function. Electricity being transferred around any property will always take the path of least resistance to the earth. Damage to live or neutral wires could increase the risk of electrocution as the electricity may pass through the human body along its path to the earth. However, this is prevented by the green and yellow earth wire as it effectively earths the electrical appliance.
There should be some warning signage indicating installations featuring circuits and/or fixed cables and wires of mixed colours. This warning should be very clearly designated on either the fuse board or the consumer unit.