Temporary veneers dark at the top, is this a bad sign? (Photo)

No need to worry about this.  It looks as though you are getting a little leakage under the temporaries and this is normal considering they are not bonded in.  Once the final veneers are placed that should no longer be present.  

Temporary veneers can change color, stain, come off and can chip. They are not like your permanent porcelain veneers with stable color.
The dark color can be due to bleeding gums and the clotting of the blood or possibly if you started with a dark root canaled tooth!
Have your cosmetic dentist check that as soon as possible to give you what you want.

Temporaries sometimes stain as they are more porous than final veneers. They pick up coffee and wine stains. This has no effect on the final result.  Once your final veneers are placed (if properly maintained) you should not have this issue.Best of luck!