You cannot do this with a textarea or input tag. However, as @naikus mentioned, you can use the contenteditable attribute. It is as follows:
<div id="mytxt" contenteditable="true">
Hello, my name is <span style="color: blue;">Bob</span>
and I have a friend name <span style="color: green;">Joe</span>.
</div>
<div id="mytxt" contenteditable="true">
Hello, my name is <span style="color: blue;">Bob</span> and I have a friend name <span style="color: green;">Joe</span>.
</div>