[Block Themes] How Do I Change The Color On The Off-Canvas Mobile Navigation?

There is no setting to do that currently, because of that you’ll have to do with custom CSS.
Please go to Superb Addons > Custom CSS and create a new CSS block with following:

@media only screen and (max-width: 599px) {
.wp-block-navigation__responsive-close,
.wp-block-navigation__container li a {
color:black !important;
}
}

You can change the word black with any color such as yellow, red, green or a color code such as #000000.