If signature keyword bold or italic then its good for ranking or beneficial for visitor. But if it is colored then not profit color keyword just attract visitor.
In signature, i don't think so, but in your contents bold and italic will be helpful. And i don't think bold and italic has direct relationship with backlinks, bold and italic just help website vistors and crawlers know website's content.
Whether you make it bold or italic or change it color, I don't think it has a direct effect. What matter most is that you put the right keyword and link at your signature.