I don't know that expression; yes, the colour is pale yellow, but oolong-tea is something between black and green tea.
Maybe it's a french thing only ?
We classify tea by "colours", so Black, GReen are the most common, but then come Red ( often considered as Rooibos even if it's a little different ), Blue, White, Yellow.
White is essentially tea buds if I recall correctly, while Blue is Oolong ( more or less ).
Yellow is a more refined Green : it's a very, very costy tea claimed to have been the tea of the Emperors ( hence the color, since Yellow was the imperial color ).
