How can i validate the input from a html5 Datalist?

Try this:

<input type="text" list="colours" id='txt'>

And on form submit you can check:

var val = $("#txt").val();

var obj = $("#colours").find("option[value='" + val + "']");

if(obj != null && obj.length > 0)
    alert("valid");  // allow form submission
else
    alert("invalid"); // don't allow form submission

You can do this with HTML5 validation using pattern. It's easier if you're populating your datalist with some sort of template, but it would look something like this (Note that you would need additional code to handle the validation - I just added very simple CSS to display the validation state)

It's worth noting that this automatically blocks form submission and provides proper semantics for accessibility and other standards-compliant interoperability.

input:valid {
  border: 1px solid green;
}
input:invalid {
  border: 1px solid red;
}

<input type="text" list="colours"
pattern="^(Red|Blue|Green|Black|White)$"
>

<datalist id="colours">
    <option value="Red" data-id="1">
    <option value="Blue" data-id="2">
    <option value="Green" data-id="3">
    <option value="Black" data-id="4">
    <option value="White" data-id="5">
</datalist>