The standard caution symbol warning pictogram is a yellow triangle with an exclamation point in the center. It is used on equipment to warn of possible dangers to operators and bystanders. It is also used in computer programs to alert the user to important messages, such as warnings regarding deletion of files. A variation of the design uses a red triangle to call more attention to the sign.
. This WIkipedia article has lots of information on the subject: http://en.wikipedia.org/wiki/Warning_sign.
