Will zoom whitening deliver results on teeth that had a previous root canal done?

Unfortunately, Zoom Whitening and similar external teeth whitening treatments are generally less effective on teeth that have undergone root canal treatment, especially when the tooth has darkened due to internal discoloration. This is because the discoloration occurs from the inside of the tooth rather than surface stains.
However, there are alternative treatments specifically designed for root canal-treated teeth:
Internal Bleaching: This involves placing a bleaching agent inside the tooth, which can help lighten it from the inside out. This is usually more effective for darkened teeth that have had root canal treatment.
Veneers or Crowns: If internal bleaching doesn’t achieve the desired result, or if the tooth is structurally compromised, placing a veneer or crown on the tooth can restore both its color and strength.
Consult with your dentist or a cosmetic specialist to see which option might be best for you. They can guide you on whether internal bleaching or another approach would give you the result you're looking for.

Tooth discoloration is a natural phenomenon following root canal treatment. Tooth whitening can be helpful but the process is different than routine whitenings. Instead of material being applied on the surface of the tooth, internal bleaching should be done. Dentist will drill an access hole from the back of the tooth, and apply the whitening internally. This is called internal bleaching. It may or may not be successful but certainly is the most conservative way to improve tooth discoloration. Your plan B would be porcelain veneer or crown.