[javascript] How do I disable TextBox using JavaScript?

Form elements can be accessed via the form's DOM element by name, not by "id" value. Give your form elements names if you want to access them like that, or else access them directly by "id" value:

document.getElementById("color").disabled = true;

edit — oh also, as pointed out by others, it's just "text", not "TextBox", for the "type" attribute.

You might want to invest a little time in reading some front-end development tutorials.