Stealth is a bonus for Mono players only. The player has to avoid all grey blocks to get a 30% bonus (20% in Audiosurf 2.)
The strategy to this is to look ahead in the song and try not to get yourself into the side filled with grey blocks. At times, it can be a choice between a big combo or getting stealth. In Audiosurf 2, using the minion ship can be a big help in getting hard to reach cars, but minimizing its use can lead to inadvertently picking up grey cars. Try to avoid usng it in areas that are highly congested with greys unless you are confident in your timing.