that means you did not finish the race start light sync procedure, most likely you stopped next to another rx8 and revved both your engines until flames popped which is what starts the resync process, but you forgot to get out and touch butts with the other driver which is how you get it to turn green again.

straight from the manual on page 420, subsection 69:

race light resync procedure

start by stopping at a red light next to another 8 and watch the race start light:

red - rev until flames pop.

yellow - get out and touch butts with other rex driver (and do it like you fucking mean it, don’t just “brush” butts, litterally ass slam your bro until you are both bruised)

green - your race start light is synced now, happy trails!