Final answer:
The correct action when a flag is carried past by a color guard at a parade is to stand at attention and salute, reflecting military customs and respect for the nation's flag.
Step-by-step explanation:
When attending a parade in uniform and the flag is carried past you by a color guard, the correct action to take is to stand at attention and salute. This is a sign of respect and protocol for the flag. It is a tradition that dates back to various military customs, representing loyalty and patriotism to the country the flag stands for. Marching in step with the color guard is not appropriate unless you are a part of the color guard. Ignoring the flag or waving enthusiastically are not respectful actions in this context.