Why do I still read voltage on outlet when I turn off breaker?
Using a Digital meter you can pick up phantom voltage fairly easily - an analog meter typically not so. Reading 2.1V  is typical of a line that crosses any other circuits. Regardless of that reading - ALWAYS work on the lines as if they are hot! You never know who has done what - if the home was owned by someone else - maybe they did something 'creative'. Even some ' pro's ' if they are not followed up on might do something that maybe it is in code but not really what should be done. 

Now it sounds like you are replacing that GFCI outlet- make sure you follow the instructions of the new GFCI outlet - your neutral is very important - of course so is your ground. Connect them correctly. In the USA White is neutral, black is hot and green (or un-insulated copper) is ground. The color of the Jacket that contains all three wires is not of any significance.

As an FYI:
Your yellow wire as you call it ... You should have what is often called Romex wire - which is a collection of conductors each insulated in their own insulation material (in their own colors.) All contained with in a jacket assembly. See an example at this link.

https://www.homedepot.com/p/Southwire-1-000-ft-12-2-2-Solid-Romex-SIMpull-CU-NM-B-W-G-Wire-55048401/202528018