Can a Veneer Be Darkened?

There is no good solution for your short term problem of  your veneer being too light.  You cannot stain the porcelain to be darker.  If you have some bleaching trays at home, you could try to whiten the rest of the teeth for 2-3 nights and hopefully the other teeth would lighten making the shade difference less noticeable.  The only other option would be for your dentist to remove that veneer immediately and place a new temporary that would match the other teeth better.  

Getting a single front tooth to match the adjacent teeth is VERY difficult. Thankfully, once done, the color stability is such that it won't change. Unfortunately, if it doesn't match, there is nothing that can be done and must be done over.
Normally it takes 2 visits, one to remove the old restoration and take an impression to send to the lab, then the second one to bond it in place. Simply calling a lab and ordering another skips a very important step.
As far as darkening the tooth until later. There really isn't anything that I am aware of.

I'm sorry to hear about your experience.  As noted by Dr. Timmerman, doing one tooth is the hardest procedure in cosmetic dentistry.    

To answer your question- NO, you can't darken your veneers.  In addition, I'm concerned that they are just "ordering" a new one.  I've done over 5000 veneers and I've yet to be able to just "order a new one".   Porcelain veneers are adhesively bonded to the underlying tooth structure.  One that is going to change ever so slightly when the old (dark one) is removed.  Thus, this would require a new impression for precision fit. 

Next veneer, ask to see it in with "try-in paste" so you can approve the look, size, contour, and COLOR of the tooth prior to the bonding process.  

I hope this helps. 

I often stain my temporary veneers to match adjacent teeth.  You can bring the value or brightness down by adding some yellow, gray, or brown depending on the color of the adjacent teeth.  How can YOU do this?  Fist I would assume that this veneer is going to be cut off, so we have nothing to lose by trying to bring its value down.  I might try to find some light brown or gray nail polish and paint it on the surface.  Be subtle.  It doesn't take much.  It this works but it doesn't stay, I would roughen the surface with an Emory board.  Let me know if you try and this works.

The color of a porcelain veneer can not be changed after it is cemented.  If you are not happy with the color it should be re-done for a perfect match.  Veneering one tooth can be a challenge.  I would recommend going to the lab and have the ceramist who is making the veneer take a custom shade.