Hello,
I am very new to using the VEX shaft encoders with Cortex. For a competition robot, I need the optical shaft encoder to start and stop counting when certain conditions are true. I'm not really sure exactly how to put this into code. I put some pseudo code below for further understanding. Thanks in advance for any help!

Please note: I am using C, not natural language. And I am referring to the actual sensors, not the sensors built in with the motor.

if the line follower sees a white surface
start counting

if the line follower sees a black surface
stop counting
and save the value of the pulses counted

You got it almost right with your pseudocode.