The yellow highlight means there is a pending paycheck for the employee, which in turn means that you (or someone) has started working on a new paycheck by entering hours for the employee in the create paychecks window. Such changes are remembered even if the window is closed and then re-opened at a later time.
On either the first or second slides of the create paycheck window, you can click the employee name to see the paycheck details - in order to confirm they're correct.
Or, you can always start over by right-clicking the a yellow- highlighted employee name and then picking Revert Paycheck. Do this on the first slide of the create paychecks window.
