There are some variations depending on the type of track event. But in general, these colors are used at most events and mean the same thing. Before any track event, a driver's meeting is held which covers that days flag locations on the track, as well as the colors they will use. These are important for your safety and the safety of others on the track.

6.11.2. Meaning of Each Flag:

A. GREEN FLAG (Solid Green)
A race is under way the instant the green flag is displayed. This flag shall normally be in possession of the Starter only, and shall not ordinarily be displayed at the flag stations around the course. When displayed, the green flag indicates that the course is clear.

B. YELLOW FLAG (Solid Yellow)
STANDING YELLOW -- Take care, Danger, Slow Down, NO PASSING FROM THE FLAG until past emergency area.
WAVED -- Great Danger, Slow Down, be prepared to stop -- NO PASSING FROM THE FLAG until past emergency area.
DOUBLE YELLOW, DISPLAYED AT ALL STATIONS — Indicates the entire course is under a yellow condition. SLOW DOWN, NO PASSING.

This flag condition may be used with or without a Pace (Safety) Car, including pace lap(s). Cars may carefully pass emergency vehicles. Cars may also pass other cars that are disabled or otherwise cannot keep the pace as signified by a raised arm on the part of the driver of the off-pace car (see Section 6.3.2.).

NOTE: A driver may encounter several flags before reaching the emergency area. The requirements are still the same “SLOW DOWN, NO PASSING.”

C. BLUE FLAG (Blue with Diagonal Yellow Stripe)
Another competitor is following you very closely or is trying to overtake you. This flag may be displayed standing or waving, depending upon the speed with which you are being overtaken.

D. SURFACE CONDITION FLAG (Yellow with Vertical Red Stripes)
Take care. Oil has been spilled, or a slippery condition exists, or debris is present on the racing surface. This flag is displayed standing.

E. WHITE FLAG (Solid White)
Caution – you are approaching a slow moving race car (e.g., with mechanical trouble), ambulance, or other emergency vehicle on the racing surface. Take care. This flag shall be shown standing for two (2) flag stations prior to the vehicle. A standing white flag shall also be displayed during the first lap of a practice or qualifying session to indicate the location of the flagging stations.

F. BLACK FLAG (Solid Black)
CLOSED BLACK FLAG (Furled) Pointed or shaken at an individual car from the Starter’s stand (optionally, accompanied by a number board indicating the car number): WARNING! You have been observed driving in an unsafe and/or improper manner. If the action continues, you shall be given an OPEN BLACK FLAG.

BLACK, OPEN - Displayed from the Starter’s stand, and accompanied by a number board indicating the car number: Proceed directly to the pits and the location designated by the Chief Steward or event Supplementary Regulations for consultation with Officials. DO NOT TAKE ANOTHER LAP. NOTE: This flag and accompanying number board may GCR - 586. Racing Rules and Procedures be additionally displayed at another station location elsewhere on the course.

BLACK, OPEN, DISPLAYED AT ALL STATIONS - The session has been halted. Practice/qualifying/racing has stopped and all cars shall proceed directly to the pits. This flag condition shall be accompanied by an ‘ALL’ sign displayed at the Starter’s stand and the sign may be repeated at stations located elsewhere on the course. If the session/race is restarted, it is done under the provisions of Section 6.5, Restarts.

NOTE: THE BLACK FLAG CAN ONLY BE DISPLAYED BY ORDER OF THE CHIEF STEWARD AS RELAYED THROUGH RACE CONTROL.

G. MECHANICAL BLACK FLAG (Black with Orange Ball) There is something mechanically wrong with your car. Proceed to your
pit or designated black flag area at reduced speed.

H. CHECKERED FLAG (Black and White Checks)
You have finished the race (or practice/qualifying session). Continue cautiously to the pits.

I. RED FLAG(Solid Red)
Displayed at each station and on the Starter’s stand - EXTREME DANGER - THE SESSION HAS BEEN STOPPED. Come to an immediate, controlled stop at the side of the race track (indicated by an official at that location or as specified in the event Supplementary Regulations). When released by an Official, proceed cautiously to the pits.

The above flag review is from the SCCA rulebook 2009 and is used as a reference for general flag usage at public and private track day events. Always join the drivers meeting for updated flag information for that day as each event is run slightly different.