On the timing screen there is a legend at the bottom.
It says:
White = most recent
Green = personal best
Purple = session best
But, there's also yellow times.
Maybe I'm just really thick and it's very obvious what they are, but I can't figure it out.
Just what do the yellow times represent?
These are the colour codes from the official F1 website live timing system hope it is same in the game.
Yellow: Default display colour.
Red: Indicates cars exiting and entering the pits. When a car leaves the pits the word OUT is displayed in the time column. The car number and driver name appears in red until that car goes through the first sector when it will revert to its normal colour. When a car enters the pits the words IN PIT are displayed in the time column. The car number and driver name is shown in red and reverts to its normal colour after a short period of time.
White: Indicates the most recent information available for that driver (e.g. a white sector time is the information for the sector just completed).
Green: Indicates a personal best for that driver and may relate to individual sector times as well as a lap time.
Magenta: Indicates the overall best in that session and may relate to individual sector times as well as a lap time.
Grey: Used in qualifying only. Indicates drivers who have been knocked out in the first or second qualifying phase and are therefore no longer eligible to participate in the session.
STOP: Appears in place of the sector information in the event of a car not completing that sector, indicating that in all probability the car has stopped on the circuit.
OUT: Appears in place of the sector information to indicate that a car has just exited the pits.
IN PIT: Appears in place of the sector information to indicate a car has just entered the pits. A line showing the personal best time in each sector for that car in the current session is displayed in yellow after the car has been in the pits for 15 seconds.
Hope it helps.