6% of the cement only? that's a serious paste failure you have there. old cement? overloading of color? with a standard oxide type pigment there should no set retardation at a 6% loading of your cement. the Tech sheet for that pigment indicates you should not use that product with any "cement substitutes" like fly ash and slag. which is also interesting.
I'd be a little Leary of that colorant. just bite the bullet and order the right stuff already.