The printers have the ability to print on both sides of a page whether in colour or black and white, and of A4 or A3 size.
If you want to print on both sides of an A4 or A3 page that has colour on it you need to select the printer A4/A3 colour from the list and then choose 'Print on Both Sides' from the options under the list of printers.
Your job will then print on both sides and you will be charged accordingly.
A4 colour print both sides = .15c per double-sided page
A3 colour print both sides = .24c per double-sided page