That's much easier by using a fake visible link:
google.com.
In my opinion, This is more dangerous than OP mentioned. If I hover over the link, I know I can see the original link on your screen's bottom-left corner. But, This is another easy way to make people fool.
No it doesn't work, either green or blue URLs will be changed to forum's background color. So you can't differentiate the URLs is Bitcointalk URLs or non Bitcointalk URLs.
Another change can prevent it. If theymos put some restriction/limit on it, like signature space, It can be prevented. If I am not wrong, Only Legendary members' signatures space can use the background color. He can put some restrictions on coloring URLs, too (In my opinion, Coloring URLs should not be allowed for anyone) like Signature space. Still, I believe your suggestion is easier to implement.