Unless you have an incredibly long length of yarn to work with, the chances are that you are going to have to a) add in a new ball of yarn or b) change to another colour or yarn.
The best secret to success here is to – join your new yarn at the beginning of a row.
Changing yarns mid row rarely goes unnoticed. At the beginning of the row, change your yarn, but make sure you leave a long enough tail to weave in the ends.