Ubuntu : Trouble printing...

Hello !
I've a problem that puzzle me.
I own a colour laser printer. Lucky me. The printer was cheap but the toner are not. It is a Brother HL-3070CW. It is plugged on my network with a DHCP permanent lease (same IP address).
As the colour toners are very expensive and not so long lived, I've defined two printers in the system, at first using the system panel. The first one is called "BrotherBlackAndWhite" and the second "BrotherColour"...
I've set the default options for the first printer to B&W and for the second to colour. I made the default printer to the B&W one.
If, for example, in Evince I open a PDF document containing colour pages and I try to print one page, the default printer is selected. And the page prints in colour... If I want it to print in B&W I've to manually go to the colour tab and select No for colour, etc... for the page to print in B&W.
This is very aoying and was working before (before the 14.04 LTS version). Now it is not working either in 14.04 nor 16.04. I've tried to reset the options using CUPS Web interface, but it did not change anything.
I would be very delighted and grateful for any proper solution to this PITA because all my attempts to make it work have pitifully failed.
Thanks for your help.