According to the FCOM:
"The DEST EFOB field will turn to amber, if the EFOB at destination becomes less than the MIN DEST FOB value displayed on the FUEL PRED page. "
The way contingency fuel is handled by the FMGS on the ground and in the air is playing a role here but I'm not sure of the explanation. Maybe someone else can add something?
However, there is a kind of "update lag" when it comes to the EFOB colour. Once it goes amber, it needs a good reason to go back green. Example, let's say your EXTRA fuel goes to -0.2 because of an incorrect route or long SID/STAR. You realise your mistake and make the correction and now EXTRA fuel is showing 0 or +0.1. Well, it's not enough to turn it green again immediately. It needs a "major update" like +0.2 or +0.3 EXTRA fuel before it changes colour. I've also seen this happen on occasions where I've entered the route first and forgot about the wind information (winds aloft or average wind component). Remember that the FMGS is constantly computing predictions as you enter information. At some point, the EXTRA fuel briefly dips below zero because it didn't know about the wind earlier. We then later return to add the wind information giving us a positive EXTRA fuel figure. But we don't see it go back to green until a later stage of the flight.