Not to get too personal, but is the ceiling really white white? Or has it, shall we say, faded or darkened slightly?
If it's still a pretty bright white they do have those spray cans of stuff. They shoot up at the ceiling. Then I'd take a putty knife and just slap it around a little until it looks like the rest of the ceiling.
Here's a pic of the stuff I think:
...oh no, that's not quite right. I think this is it:
I think they might even make it in "faded" white. Or just sit under your patched spot and smoke blunts all day. It's a tough job but someone has to do it.
Sorry guys, I'm sick and bored today :)