There's a scientific reason NOT to have them red and green, which is that between 6%-8% of men are red-green colour blind and can't tell the difference so it's useless for them.
From the user POV using green and red either way round in a buy/sell choice doesn't seem to have any scientific basis.
Both could cost you money if you made the wrong choice at the wrong time, so it's not like one is more 'dangerous' than the other to the user. This may well be why your client doesn't want them differentiated as such.
However, I've noticed in my trading platform of choice (a european one) that the buy button is green and a '+' sign whilst the sell button is red and a '-' button.
Understanding Candlestick Charts for Beginners
Taking into account that, in nature at least, red is seen as 'danger', the cynical side of me says that's to give a subliminal nudge that selling and possibly taking your money and going is bad, but putting more money in for them to look after is good :-)
It'd be interesting to know if the rest of the trading platforms you surveyed have it the same way round or are swayed by the cultural/regional meanings Michael Lai pointed out depending on their target market