The UK's registration plates have changed over the years, and at the moment they must be white in the front and yellow in the back. The primary reasons for choosing this difference are to improve visibility and aid law enforcement. The white front plates make it easier for traffic officers and speed cameras to read the registration number, especially in low light. The yellow license plate, which is clearly visible in low light, improves the vehicle's visibility and safety. This system makes it incredibly easy for police officers and motorists to find and report vehicles that may be involved in moving violations or accidents.

Original Source: https://www.quora.com/Why-are-the-registration-plates-in-the-UK-white-in-the-front-while-those-at-the-rear-are-yellow/answer/Leicester-Motor