If you want to set color of text in your navbar to white, just add dark like so: <MDBNavbar dark>.

Extended example:

<MDBNavbar color="default-color" dark expand="md" style={{marginTop: "20px"}}>
    <MDBNavbarBrand>
      <strong className="white-text">MDBNavbar</strong>
    </MDBNavbarBrand>
    <MDBNavbarToggler onClick={this.toggleCollapse("navbarCollapse3")} />
    <MDBCollapse id="navbarCollapse3" isOpen={this.state.collapseID} navbar>
      <MDBNavbarNav left>
        <MDBNavItem active>
          <MDBNavLink to="#!">Home</MDBNavLink>
        </MDBNavItem>
        <MDBNavItem>
          <MDBNavLink to="#!">Features</MDBNavLink>
        </MDBNavItem>
        <MDBNavItem>
          <MDBNavLink to="#!">Pricing</MDBNavLink>
        </MDBNavItem>
        <MDBNavItem>
          <MDBDropdown>
            <MDBDropdownToggle  nav caret>
              <div className="d-none d-md-inline">MDBDropdown</div>
            </MDBDropdownToggle>
            <MDBDropdownMenu className="dropdown-default"right>
              <MDBDropdownItem href="#!">Action</MDBDropdownItem>
              <MDBDropdownItem href="#!">Another Action</MDBDropdownItem>
              <MDBDropdownItem href="#!">Something else here</MDBDropdownItem>
              <MDBDropdownItem href="#!">Something else here</MDBDropdownItem>
            </MDBDropdownMenu>
          </MDBDropdown>
        </MDBNavItem>
      </MDBNavbarNav>
      <MDBNavbarNav right>
        <MDBNavItem>
          <MDBNavLink className="waves-effect waves-light" to="#!"><MDBIcon brand icon="twitter" /></MDBNavLink>
        </MDBNavItem>
        <MDBNavItem>
          <MDBNavLink className="waves-effect waves-light" to="#!"><MDBIcon brand icon="google-plus" /></MDBNavLink>
        </MDBNavItem>
        <MDBNavItem>
          <MDBDropdown>
            <MDBDropdownToggle nav caret>
              <MDBIcon icon="user" />
            </MDBDropdownToggle>
            <MDBDropdownMenu className="dropdown-default" right>
              <MDBDropdownItem href="#!">Action</MDBDropdownItem>
              <MDBDropdownItem href="#!">Another Action</MDBDropdownItem>
              <MDBDropdownItem href="#!">Something else here</MDBDropdownItem>
              <MDBDropdownItem href="#!">Something else here</MDBDropdownItem>
            </MDBDropdownMenu>
          </MDBDropdown>
        </MDBNavItem>
      </MDBNavbarNav>
    </MDBCollapse>
  </MDBNavbar>

Best,

Jakub