This is referred to as "Blonding" - the colour transition of an espresso pour from dark brown and striped to a light, uniform pale blond. This normally occurs in the last third of the pull and is a signal to end the pour. This overly-blond portion of an espresso is thin, nearly flavourless, and if allowed to continue will dilute the body and taste characteristics of a shot.
Check out the following article for an in depth look at how to get the best results from your pods: