Some tattoos can interfere with the plethysmograph sensor in Apple Watch

So does the Watch run into problems with wrist tattoos? Yes and no. I spent an hour today testing the Watch's sensor reading on multiple tattoo colors, and have indeed managed to replicate some of the issues Reddit and Twitter users were having.
Dark, solid colors seem to give the sensor the most trouble
When it comes to patterned or variegated wrist ink, however, we couldn't reproduce the misreadings or errors other users have been seeing. This may entirely depend on the type and design of tattoo, however, along with ink and skin saturation.

Serenity Caldwell | iMore