I can confirm that hex values don't show up in auto-complete suggestions and agree that this would be a nice feature. I tinkered for an hour and wasn't able to figure it out. I'll have to deepen my understanding of how symbol indexing works in Sublime to implement this.

Thank you!

I found out that any custom completion inside {} is ignored. Sublime only lists the values given by your CSS3 package while inside the brackets.
Autocomplete of hex values outside the brackets is working!

Any Update?

Sorry, I haven't had time to work on this yet. I'm gonna work on it this weekend, though. It's definitely on my TODO list.

UPDATE: While I was working on improving the symbol indexing, I discovered that the new dev builds of Sublime Text 3 introduce an entirely new system for syntax highlighting. It's a much better system than the old TextMate XML-based garbage. Since symbol indexing is somewhat coupled to the scope names assigned by the syntax highlighter, I've decided to put off any more work on symbol indexing while I rewrite the highlighter from scratch. That's going to take a long time (sorry!), but it should fix a lot annoying bugs. I will keep this issue open and come back to it when the new highlighter has stabilized.

Any updates on this topic?