From the official FIM Rule book
Official flag signals will be given by means of a flag measuring 750 mm high by
600 mm wide as follows:
Signal Meaning
Red flag All riders must stop.
(All false starts must be indicated by waving a red flag)
Black flag and a board with a
rider’s number on it
Rider in question to stop.
Yellow flag, held stationary Danger, ride cautiously.
Yellow flag, waved Immediate danger, prepare to stop, no
overtaking.
Blue flag, waved Warning, you are about to be lapped.
(The blue flag must be used by supplementary flag marshals, specialised for
this flag only)
Green flag Course clear for the start of the race.
(The green flag can only be used by an Official during the start procedure)
Black and white chequered
flag
End of the practices, the qualifications, the
warm-up, the race.
The Pantones for the colours are as follows:
Black: Pantone Black C
Blue: Pantone 286C
Red: Pantone 186C
Yellow: Pantone Yellow C
Green: Pantone 348C
White: Pantone White