red

HTML

<script src="http://knockoutjs.com/js/knockout-2.1.0.js"></script>
<div id="formData">
   Input - <input type='text' data-bind="redNegativeValue : test" /> <br />
   Input - <input type='text' data-bind="test" /> <br />
   Result- <span data-bind="text: test" ></span>
</div>

CSS

.red {color: Red}

JavaScript

ko.bindingHandlers.redNegativeValue = {
    init: function(element, valueAccessor, allBindingsAccessor, viewModel) {
        ko.bindingHandlers.value.init(element, valueAccessor, allBindingsAccessor, viewModel);
    },
    update: function(element, valueAccessor, allBindingsAccessor, viewModel) {
        ko.bindingHandlers.value.update(element, valueAccessor, allBindingsAccessor, viewModel);

        var value = valueAccessor();
        var val = ko.utils.unwrapObservable(value)
        if (val && val < 0) {
            $(element).addClass('red');
        } else {
            $(element).removeClass('red');
        }

    }
};
viewModel = {};
viewModel.test = ko.observable(-1);
ko.applyBindings(viewModel);