Invoice output in yellow and red color why?

Hi Experts,

Kindly clarify why and when exactly the outputs are in yellow and red color and what we need to do change the from Yellow or red to Green.

Eleventhough output in red/ yellow color , i am able to view the output print preview and able to taking printout for the documents.

I really searched allot in Google, but i could not get the relevant answer.

thanks in advance.