I can offer great products like Color by Robert Craig, suggestions, tips and helpful information, but many times I do recommend a professional. I think you should see a professional if:

• you have never colored your hair, want a major change and expect a near perfect result without a little experimenting
  
• you have colored your hair and it didn't turn out anything like you expected
  
• you want a very natural highlighting or lowlighting
  
• your hair is very, very damaged