What error occurs if you forget to indent after a colon in Python?

When you forget to indent after a colon in Python, an IndentationError occurs. This error is specifically related to the way Python uses indentation to define the blocks of code. When a colon is used in constructs like functions, loops, or conditionals, Python expects the next line to be indented to signify that it is part of the block initiated by the colon. If the indentation is missing, Python cannot determine the structure of the code, resulting in an IndentationError.

This behavior exemplifies Python's reliance on whitespace to organize code logically, making indentation crucial for not just readability but for the correct execution of the program. Being able to identify and correct this error is fundamental in writing efficient and error-free Python code.