Positive numbers blue, negative red?

Please, how to achieve that I get positive numbers blue and negative red in that program:

<!DOCTYPE html>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<p Colorme if {(nr1>0&&nr2<0)  {color:red;}}

<p>Click the button to demonstrate the prompt box.</p>
<button onclick="myFunction()">Try it</button>
<p>Nr #1: <b><span id="nr1"></span></b></p>
<p>Nr #2: <b><span id="nr2"></span></b></p>

<p>Add: <b><span class ="barva" id="sestevek"></span></b></p>
<p>Diff: <b><span style="color:blue" id="razlika"></span></b></p>


function addition(nr1, nr2) {

	var sum = nr1 + nr2;
	return sum;

function differentiation(nr1, nr2) {

	var diff = nr1 - nr2;
	return diff;

function myFunction()
	var nr1=prompt("Please enter nr1","number1");
	var nr2=prompt("Please enter nr2","number2");

	nr1 = parseInt(nr1);
	nr2 = parseInt(nr2);

	document.getElementById("nr1").innerHTML= nr1;
	document.getElementById("nr2").innerHTML= nr2;
	document.getElementById("sestevek").innerHTML = addition(nr1, nr2);
	document.getElementById("razlika").innerHTML  = differentiation(nr1, nr2);