Can I type text in color in the text box?

If you are asking changing the font color of input text, it is possible with JotForm as you see below:

 

If you have any question or issues, please do not hesitate contacting us. We will be glad to assist you.

Is it possible for only one text box?
...

Yes, it's quite feasible to apply text color (and other styling changes) to just certain text boxes. That's what CSS's ID selectors are for. For instance, in this demo entered the text of just the second text box is red. That was accomplished by going into the designer

and clicking the CSS tab

and then double-clicking the second text box. That action added an empty rule to the tab's work area (indicated by the second arrow) for me to enter a declaration of color: red;, as shown below:

#input_3 {
   color: red;
}

Since the ID of the second field is input_3 (please see this guide) the rule affects just that field.

If you need further help with this, we'd be happy to provide it.

Cheers