You can add the following to your scss file that will show a border color of red. For testing I also added a background color of red -
::ng-deep [dir=rtl] .textFieldInvalid.mat-mdc-form-field:hover .mat-mdc-text-field-wrapper .mdc-notched-outline__leading,
::ng-deep .textFieldInvalid.mat-mdc-form-field:hover .mat-mdc-text-field-wrapper .mdc-notched-outline__leading {
border-color: red;
background-color: red;
}