Hi Neil,
To change the hamburger menu icon for the Twenty Twenty theme, you can add some custom CSS (Customizer > Additional CSS, change the #ff0000 value with your own color):
body:not(.overlay-header) #site-header .toggle {
color: #ff0000;
}
I hope the above is useful to you.
If you enjoy Twentig, please rate it. It would really help me out 🙂
Tom