Well AFAIK some of the new colours do not have Vallejo equivalents.
Dry colours like Hellion Green definitelly does not have as Vallejo does not have dry colours in range.
But you might want to try this - find a "splat picture" of the colour you want on GW page, put it into Gimp or Photoshop, find its colour value and then try to find similar colour value in the Vallejo Model and Game ranges - the charts can be downloaded from Vallejo website.
I know its bit too much work, but if you REALLY want to find an equivalent... :-)