Unfortunately the markup for Google+ (and Google Talk by extension) is fairly limited.
+name to tag someone.
Drag and drop
Add photos, videos, or links to your posts by dragging them directly to the share box. To add a link, drag the icon next to the URL from the address bar to the share box.
- Use underscores to italicize. _ italics_ → italics
- Use asterisks to bold. * bold* → bold
- Use hyphens to strikethrough. - strikethrough - →
- Compound formatting: *_ bold italic_* → bold italics (make sure the nesting mirrors on each end)
Mention your friends
Use + or @ to add someone to the conversation and link to their profile. +Natalie Villalobos or @Natalie Villalobos → +Natalie Villalobos learn more about mentioning people in posts.
Post from the +1 button
When you click the +1 button across the web, you'll have the opportunity to create a post with a link to the page you +1'd. Learn more about the +1 button.
Share box chip colors
In the sharebox, groups of people you share with appear as either a blue or green "chip", like "public" or "extended circles". When a chip is blue it means that you've selected the people with whom that content is being shared. For instance, your Family circle is blue because you decide who you place in that circle. When a chip is green, it means that it may contain people you haven't added to your circles. Your extended circles, for instance, appear as a green chip because some of these people likely aren't in your circles.
Reference: Share a photo, video, link, or event, section "Posting tips"