As i've pointed out else where, especially to be considerate for our colour blind friends (depending on what group they are in), We should be consistent with out colour schemeing, Although this should be the only usage in core (as far as I know), We have seem to have started using blue for buttons to indicate "Ok"/"Go" so that shouldn't be mixed in a display with Green which is commonly used to indicate a similar meaning.
(Having blue for those buttons, though, Is a completely different discussion)