Possible redundant code improvement
1)

Take advantage of javascript's truthy and falsey values.

Boolean(undefined); // =&gt; false
Boolean(null); // =&gt; false
Boolean(false); // =&gt; false
Boolean(0); // =&gt; false
Boolean(""); // =&gt; false
Boolean(NaN); // =&gt; false

Boolean(1); // =&gt; true
Boolean([1,2,3]); // =&gt; true
Boolean(function(){}); // =&gt; true


More here: http://james.padolsey.com/javascript/truthy-falsey/

if(monmpf == "") => if(!monmpf). 

if(minutesCharset == true) => if(minutesCharset)

if(monhpf.length &gt; 0) => if( monhpf.length )

2)

You can refer to the current element by using $(this).

$('#monmpf').keyup(function () {
// el refers to $("monmpf")
var el = $(this);


3)

if (monhpf.length &gt; 0) is redundant since you already checked the string value with this, if(monmpf == "").

4)

You're using too many global variables, such as emptyField, invalid, green, red, orange, etc.
Try wrapping the variables inside a meaningful namespace, like 

var COLORS = { red: "red", green: "green" };
var MESSAGE = { invalid: "Wrong", emptyField: "" };


5)

mondayErrors? I think you should rename this variable.

6)

Break up functions that are longer than 8-12 lines of code into smaller ones.

With all that said, here's what I came up with.

var getRequestObject = function (name, value) {
    var obj = {};
    switch (name) {
        case "monhpf":
            obj.url = 'functions/hoursCharset.php';
            obj.paramObj = {
                hoursPHP : value
            };
            break;
        case "monmpf":
            obj.url = 'functions/minutesCharset.php';
            obj.paramObj = {
                minutesPHP : value
            };
            break;
    }
    return obj;
};
var getPostFunc = function ($el) {
    return function (isCharset) {
        mondayErrors = !isCharset;
        if (isCharset) {
            $el.css('border-color', green);
            $('.mondayErrors').hide();
        } else {
            $el.css('border-color', orange);
            $('.mondayErrors').text(invalid).show();
        }
    };
};
$('#monhpf, #monmpf').keyup(function () {
    var $el = $(this),
        request,
        value = $el.val(),
        id = $el.attr('id');

    if (value) {
        request = getRequestObject(id, value);
        $.post(request.url, request.paramObj, getPostFunc($el));
    } else {
        $el.css('border-color', red);
        $('.mondayErrors').text(emptyField).show();
    }
});