Some good information;
About Track Grip and Rubber Level
Rubber level depends on how many cars are circulating on the track.Also, the later the session is in the weekend, the more simulated rubber it will have from simulated support races.
As a rule of thumb, Friday has low grip levels, Saturday is already high and Sunday is already usually optimum grip.
Remember that Optimum grip means a grippy ideal line, but the off-line will have marbles that make it slippier than how a less grippy track off-line would be.
Marbles are visually represented and also produce an audio effect when driving on them, so you can have a good indication in terms of what you can expect.
The sequence of track grip levels are:
GREEN - no rubber, no marbles -> most slippery but grip is evenly spread.
FAST - medium rubber, medium marbles -> grippier ideal line, slippier than normal outside line.
OPTIMUM - most rubbered in, ideal line has maximum grip, outside line is very slippery.
GREASY - slippery track with low wetness level, byproduct of the rubbered line becoming wet.
DAMP - low wetness level, usually still somewhat manageable on slicks unless worsening.
WET - high wetness level with potential puddles, wet tyres recommended unless improving.
FLOODED - lots of standing water on the track, no drying line in sight.
The three dry status indicators (Green, Fast and Optimum)
each cover a range, they are not a specific % of grip level, and also the different grip statuses have differing ideal line/outside line correlations.
If you imagine a range of 0-100, 0 being the least amount of grip, and 100 the highest, green, fast and optimum each cover a 20-40% range within there. So the lower end of "Fast" when it has just tipped over "Green" and the higher end where it's almost "Optimum" can produce different lap times, while still showing the same "Fast" status.
Very important: the more grip the ideal line has (i.e. the more rubbered-in it is), the more rubber debris you have on the non-ideal line, so green track has a low but evenly distributed grip level on all the tarmac surface, while optimum has a relatively big drop-off if you skid off the ideal line into the rubber debris.
Bottom line, the 3 indicators cover a range each, and the higher the grip is, the bigger the dropoff is on the outside line.
Found in this guide from official Kunos support forum;
Newcomers guide to ACC