In general it's not a good practice to style with !important attribute in CSS. You should try to avoid it.
You will need to find in your browser inspector which CSS rules to color your link on hover.
You can find information about how CSS rules overwrite them selves and what rule get's specified in the end: https://developer.mozilla.org/en-US/docs/Web/CSS/Specificity
E.g.: You already have a hover state style for the link which could override your white color style:
.navbar-nav a:hover, .dropdown-menu>li>a:hover {
color: #a4cb9a !important;
}
in http://beta2018.bedcentregrimsby.co.uk/wp-content/themes/BedCentre2/css/bedcentregrimsby.css?ver=4.9.8 at line 1006