<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<input type="text" name="" id="" class="input">
<input type="text" name="" id="" class="input">
<input type="text" name="" id="" class="input">
<input type="text" name="" id="" class="input">
<input type="text" name="" id="" class="input">
<input type="text" name="" id="" class="input">
<input type="text" name="" id="" class="input">
<input type="text" name="" id="" class="input">
<input type="text" name="" id="" class="input">
<input type="text" name="" id="" class="input">
<button class="btn">Отправить</button>
</body>
<script>
let btn = document.querySelector(".btn")
let inputs = Array.from(document.querySelectorAll(".input"))
btn.style.background = "tomato"
inputs.forEach(input => {
input.addEventListener("input", function(){
let newArray = inputs.filter(function(input){
return input.value == ""
})
if(newArray == ""){
btn.style.background = "skyblue"
}
else{
btn.style.background = "tomato"
}
})
});
</script>
</html>