Veneers too white. If they are thinned out, will this make my veneer weaker? Will it even work?

I'm sorry to hear that you're not completely satisfied with the color of your porcelain veneers. It's important to feel confident and happy with your dental treatments. Regarding your concerns:
Thinning Out Veneers: Thinning out porcelain veneers to allow the natural tooth color to show through is a technique some dentists use to adjust the color. However, this process can indeed make the veneers more fragile. Porcelain veneers are already quite thin (usually around 0.5 mm to 0.7 mm) and further reduction in their thickness can compromise their strength and durability.
Effectiveness of the Procedure: The effectiveness of thinning out the veneers to alter the color can vary. It depends on several factors, including the original thickness of the veneers, the color and condition of your natural teeth, and the skill of the dentist. It's a delicate balance to achieve the desired color without compromising the integrity of the veneer.
Other Options: Before proceeding with thinning the veneers, you might want to discuss other options with your dentist. For instance, sometimes, adjusting the color of the bonding cement can make a difference. Another option might be to replace the veneers with ones of a more suitable color, though this is more invasive and costly.
Consultation and Second Opinion: If you're unsure or uncomfortable with the proposed solution, it's always wise to seek a second opinion from another dental professional. Different dentists might have various approaches and solutions based on their experience and expertise.
Future Considerations: For future dental cosmetic work, always ensure you are completely comfortable with the choice before proceeding. It's okay to take your time in making these decisions, as they have a lasting impact on your appearance and dental health.
Remember, your comfort and satisfaction are paramount. It's important to have open and thorough discussions with your dentist about your concerns and the potential outcomes of any procedure.

It is unfortunately impossible to make the teeth darkened with dark cement. You will need to replace the veneers with new ones.