Are my crowns too short & how to mask dark tooth under crown? (Photo)

Hello. Discoloration may be related to the root canal treatment and it can be managed by intracoronal bleaching or cleaning the discolorated portion and placing a post into the root to support the restoration.
The length issue can be evaluated by your lower teeth when mouth is closed. If your jaw relationship allows, upper central incisors may be a little longer.

1. If the crown is made from zirconia (specifically "porcelain-fused zirconia" is what would be used in the anterior, as it's more esthetic), it can better mask the tooth underneath it. 2. A procedure called internal bleaching may be able to be done to lighten the "dead" tooth underneath before a new crown is placed.
*All advice/any statement made here should not be taken as definitive dental advice. The ideal way to obtain definitive information is with proper diagnostic procedures during an in-person consultation.

You can definitely mask the dark tooth underneath. When I am preping a dark tooth I like to prep it a little more to provide more porcelain to mask the dark color. Also I would put a white liner before taking impressions so that the tooth has a brighter appearance when we seat the new crown. I feel like the crowns could use a little more length. Ideally, I like my two front teeth to match the length of the canines which are the sharper teeth.