Untitled

<body>
<textarea name="texto" id="texto">
</textarea>

<button class="button button2" type="button1">Texto</button>

<script>
var textarea = document.querySelector('textarea');
var button1 = document.querySelector('button');
var assinatura1 = 'TEXTO COLORIDO'
button1.addEventListener('click', function() {
    textarea.value += assinatura1;
});
</script>
</body>