I’ve come across this problem in the past with changing the color of a custom hamburger, and you’ll be glad to know it’s very simple 
Step One is to make all the hamburger lines relative:
Next, we’ll need to add a new div block to each of the hamburger lines:

Give the div block the following styles:
Position: Absolute,
Left: 0, Right: 0, Top: 0, Bottom: 0,
Background-Color: Whatever you desire,
Opacity: 0%
Screen shot below:
With that set up, we’ll need to add an interaction to the navbar.
Simply click the navbar field, go to the interactions panel and create a new interaction. When selecting the type of trigger, click Navbar. Add ‘Line Background’ to the ‘Affect Different Elements’ field. On Navbar Open, set Opacity to 100% and on Navbar Close, set Opacity to 0%:

And that should be it! Hope I explained it easily but if not I’m willing to answer your questions 
PS liking your transition of the lines from open to close 