Apply style to specific string in text area?

No, that is not possible with the default text rendering facilities Unity 3D currently provides.

What you can do is use different text objects to render each part of the string with a different material. But I'd also recommend you take a look at third party Unity plug-ins for text. I would recommend *Toolkit 2D* and *NGUI*.

I can only think of doing it by extending GUI methods to add a "sequential" GUI option or something like that, but in my opinion that is not a very good solution, only that I can´t think of anything else.

 using UnityEngine;
 using System;
 using System.Drawing;
 using System.Collections;
 using System.Collections.Generic;
 using System.Windows.Forms;
 
 public static class GUIExtension{
         public static void SequentialLabel(this GUI gui, Rect rect, List<StyledString> styledStrings){
             float recorredWidth = 0;
 
             for (int i=0; i<styledStrings.Count; i++){
                 Size stringSize = TextRenderer.MeasureText(styledStrings[i].text, styledStrings[i].style.Font.size);
 
                 GUI.Box(new Rect(rect.x + recorredWidth, rect.y, rect.width, rect.height), 
                         styledStrings[i].text, styledStrings[i].style);
 
                 recorredWidth += stringSize.width;
             }
         }
     }
 
     public class StyledString{
         public string text {get; private set;}
         public GUIStyle style {get; private set;}
 
         public StyledString(string text, GUIStyle style){
             this.text = text;
             this.style = style;
         }
     }

Note: I have a problem with autocompletion right now so probably the code needs fixing. Oh, and that is only for "one line" label, if you want more lines you will have to do a line end check.