this.state = {
      isOpen: false,
      linkRefActiveColor: "#000",
      linkRefNotActiveColor: "#fff"
    };

<NavItem>
                <NavLink href="#" onClick={this.activeLink}>
                  <FaHome
                    style={{
                      color: this.state.linkRefActive
                        ? this.state.linkRefActiveColor
                        : this.state.linkRefNotActiveColor
                    }}
                  />
                </NavLink>
              </NavItem>
              <NavItem>
                <NavLink href="#" onClick={this.activeLink}>
                  <FaNetworkWired
                    style={{
                      color: this.state.linkRefActive
                        ? this.state.linkRefActiveColor
                        : this.state.linkRefNotActiveColor
                    }}
                  />
                </NavLink>