COROS watches are not guaranteed to work over tattoos, as it depends on the color, pattern, and size of the tattoo.
COROS watches, like many other fitness and smartwatches, use optical sensors to monitor heart rate, and these sensors typically work by shining light into the skin and measuring the amount of light that is reflected back. Tattoos can potentially interfere with this process, as the ink in tattoos may absorb or scatter the light, making it challenging for the sensors to accurately measure heart rate.
The extent to which COROS works well over tattoos can vary depending on factors such as the color and density of the tattoo, and the specific design of the tattoo. In general, darker and more densely inked tattoos may pose a greater challenge for optical heart rate sensors.
If you have a tattoo in the area where the watch's sensors make contact with your skin, you may experience less accurate heart rate readings. In such cases, it's recommended to try wearing the watch on a different part of your body without a tattoo to see if the accuracy improves, or pair with an external heart rate strap.
For more information on how optical HR sensors work, please see our article: How do COROS watches measure heart rate?