How much darker can cement make veneers & crown?

I am checking in to see whether it is possible to alter veneers in a BL4/B1 at the neck tooth shade to a 1M1.5 with warmth at the top, which would be my ideal color. I won't go into the long story behind this question, but I would also like to know whether it is customary for a lab to be able to slightly darken veneers prior to their being cemented. I was told no by my dentist's office, but I had thought that would be a possibility.