What are the EDH color identity rules and how do they impact deck construction in the format?

In EDH (Elder Dragon Highlander), each deck has a color identity based on the colors of the commander. The color identity includes all colors in the commander's casting cost and any mana symbols in the card's rules text. Players can only include cards in their deck that match the color identity of their commander. This rule impacts deck construction by limiting the cards players can use, encouraging strategic deck building and creativity within the format.