The rarest colors of French Bull Dogs are the colors blue grey and black.
If you want to buy a blue/grey or black French Bulldog you'll have to pay a higher price for those rarest colors of the bulldog.
The less rare colors of French Bulldogs are the colors fawn, pied and brindle.
The rarest color of French Bulldog would be the Cream colored French Bulldog which is the most expensive French Bulldog that you can buy.
If you want a French Bulldog that is creamed colored then you can expect to pay between $5,000.000 to $10,000.00
The other colors of French Bulldogs are between $1,400.00 to $4,000.00
You'll need some good money to buy a French Bulldog although they are very good dogs.