A BEW is any coloured rabbit ( chestnut, black, opal, blue REW, himilayan, sable, tan etc..) that has two Vienna genes.
Now to get the show quality sky blue eyes it is best if the true colour of the rabbits eye hidden by the blue is dark brown. Colours like chocolate, dilute/blue, chinchilla, sable, himilayan and REW typically have lighter eyes or red and if they happen to be BEW's their eyes will turn out as a light blue or can have a reddish cast to them.
For your BEW project I recommend you use chestnut or black Vienna carriers and breed these to another chestnut or black Vienna carrier or a BEW
A chocolate rabbit is genetically - aa bb C_ D_ E_ and has two recessive genes on the A and B-locus. If you breed two chocolates together you'll get more chocolates but you could get other colurs depending on what genes are in the blank spots in their genotype and hiding.